Buying under my bid price
It definitely depends on the exchange you are trading on. I'm not familiar with Scottrade, but a standard practice is to fulfill limit orders in the order they are placed.   Most of the time, you wouldn't see stocks trade significantly under your bid price, but since penny stocks are very volatile, it's more likely their price could drop quickly past your bid and then return above it while only fulfilling a portion of the orders placed. Example 1. Penny stock priced at $0.12 2. Others place limit orders to buy at $0.10 3. You place limit order to buy at $0.10 4. Stock price drops to $0.07 and some orders are filled (anything $0.07 or higher) based on a first-come first-served basis 5. Due to the increase in purchases of the penny stock, the price rises above $0.10 before your order is filled ***EDIT*** - Adding additional clarification from comment section. A second example If the price drops from $0.12 to $0.07, then orders for all prices from $0.07 and above will start to be filled from the oldest order first. That might mean that the oldest order was a limit buy order for 100 shares at $0.09, and since that is above the current ask price, it will be filled first. The next order might be for 800 shares at $0.07. It's possible for a subset of these to be filled (let's say 400) before the share's price increases from the increased demand. Then, if the price goes above $0.10, your bid will not be filled during that time.
Your assets are marked to market. If you buy at X, and the market is bidding at 99.9% * X then you've already lost 0.1%. This is a market value oriented way of looking at costs. You could always value your assets with mark to model, and maybe you do, but no one else will.  Just because you think the stock is worth 2*X doesn't mean the rest of the world agrees, evidenced by the bid.  You surely won't get any margin loans based upon mark to model.  Your bankers won't be convinced of the valuation of your assets based upon mark to model. By strictly a market value oriented way of valuing assets, there is a bid/ask cost. more clarification Relative to littleadv, this is actually a good exposition between the differences between cash and accrual accounting. littleadv is focusing completely on the cash cost of the asset at the time of transaction and saying that there is no bid/ask cost.  Through the lens of cash accounting, that is 100% correct. However, if one uses accrual accounting marking assets to market (as we all do with marketable assets like stocks, bonds, options, etc), there may be a bid/ask cost.  At the time of transaction, the bids used to trade (one's own) are exhausted.  According to exchange rules that are now practically uniform: the highest bid is given priority, and if two bids are bidding the exact same highest price then the oldest bid is given priority; therefore the oldest highest bid has been exhausted and removed at trade. At the time of transaction, the value of the asset cannot be one's own bid but the highest oldest bid leftover.  If that highest oldest bid is lower than the price paid (even with liquid stocks this is usually the case) then one has accrued a bid/ask cost. The best ask is the lowest ask, and the best bid is the highest bid.  If the ask was lower than the bid then they crossed, and that would be a crossed market and quickly resolved. So the bid will almost always be cheaper than the ask. A heuristic is that a bid is the revenue of the stock at any given time while the ask is the cost, so the market will only ever offer a profit to itself not to the liquidity seeker. If examining the book vertically, all orders are usually sorted descending. Since the best ask is the lowest ask, it is on the bottom of the asks, and vice versa for the best bid. The best bid & best ask will be those closest since that's the narrowest spread and price-time priority will promise that a bid that crosses the asks will hit the lowest ask, the best possible price for the bidder and vice versa for an ask that crosses the best bid. I assume you're talking about a sell order, not a buy order. When you place a limit sell order, your order is guaranteed to be placed at that price or higher.  If the market is currently trading much higher than the price of your sell order, then your mistakenly low limit order will be essentially a market order, and will be filled at  the current bid price. So the only way this is a problem is if you want to place a limit sell that is much higher than the current market, but mistakenly place a limit lower than the current market. Today SPY (The S&P ETF) trades at $128. The option to buy at $140 (this is a Jan '13 call) trades for $5.  I buy the call, for $500 as they trade in 100 lots.  The S&P skyrockets to 1500 and SPY to $150. The call trades for $11, as it still has a month or two before expiring, so I sell it, and get $1100. The S&P rose 17%, but I doubled my money. If it 'only' rose 9%, to less than $140, I'd lose my investment. No, I don't need to buy the SPY I can sell the call any time before expiration. In fact, most options are not exercised, they are sold between purchase and expiration date. The obvious thing would happen. 10 shares change owner at the price of $100. A partially still open selling order would remain. Market orders without limits means to buy or sell at the best possible or current price. However, this is not very realistic. Usually there is a spread between the bid and the ask price and the reason is that market makers are acting in between. They would immediately exploit this situation, for example, by placing appropriately limited orders. Orders without limits are not advisable for stocks with low trading activity. Would you buy or sell stuff without caring for the price? "I place a trade, a limit order on a thinly traded stock. I want to buy 1000 shares at $10. The current price is $10.50. Someone places a market order for 500 shares. Another trader has a limit order for $10.10 for 400 shares. His order fills, and I get 100 at my price. I wait another day to see if I get any more shares. This is just an example of how it can work. I can place my order as ""all or none"" if I wish to avoid this." How would this trade behave IRL? I don't know how the simulation handles limit orders and bid/ask spreads to know it's feasible, but buying at 4.04 when the current ask is 8.00 seems unlikely.  That would mean that all other sell orders between 8.00 and 4.04 were fulfilled, which means that there were very few sellers or that sell pressure spiked, both of which seem unlikely. In reality, it seems more likely that your order would have sat there until the ask dropped to $4.04 (if it ever did), and then you'd have to wait until the bid rose to $7.89 in order to sell them at that price. However, that kind of swing in option prices in not unrealistic.  Options near at-the-money tend to move in price at about 50% of the change in the underlying, so if amazon suddenly dropped by $5, the option price could drop by $2.60 (from 6.66 to $4.04), and then rise back to $7.89 if the price rose $8 (which would be 1% swing and not unheard of intra-day). But it sounds like you got very lucky (or the simulation doesn't handle option trading realistically) - I've traded options in the past and have had some breaks similar to yours.  I've also had bad breaks where I lost my entire investment (the options expire out-of-the money). So it should be a very limited part of your portfolio, and probably only used for risk management (e.g. buying put options to lock in some gains but keeping some upside potential). "It depends on many factors, but generally, the bid/ask spread will give you an idea. There are typically two ways to buy (or sell) a security: With a limit order, you would place a buy for 100 shares at $30-. Then it's easy, in the worst case you will get your 100 shares at $30 each exactly. You may get lucky and have the price fall, then you will pay less than $30. Of course if the price immediately goes up to say $35, nobody will sell at the $30 you want, so your broker will happily sit on his hands and rake in the commission while waiting on what is now a hail Mary ask. With a market order, you have the problem you mention: The ticker says $30, but say after you buy the first 5 shares at $30 the price shoots up and the rest are $32 each - you have now paid on average $31.9 per share. This could happen because there is a limit order for 5 at $30 and 200 at $32 (you would have filled only part of that 200). You would be able to see these in the order book (sometimes shown as bid/ask spread or market depth). However, the order book is not law. Just because there's an ask for 10k shares at $35 each for your $30 X stock, doesn't mean that by the time the price comes up to $35, the offer will still be up. The guy (or algorithm) who put it up may see the price going up and decide he now wants $40 each for his 10k shares. Also, people aren't obligated to put in their order: Maybe there's a trader who intends to trade a large volume when the price hits a certain level, like a limit order, but he elected to not put in a limit order and instead watch the ticker and react in real time. Then you will see a huge order suddenly come in out of nowhere. So while the order book is informative, what you are asking is actually fundamentally impossible to know fully, unless you can read the minds of every interested trader. As others said, in ""normal"" securities (meaning traded at a major exchange, especially those in the S&P500) you simply can't move the price, the market is too deep. You would need millions of dollars to budge the price, and if you had that much money, you wouldn't be asking here on a QA site, you would have a professional financial advisor (or even a team) that specializes in distributing your large transaction over a longer time to minimize the effect on the market. With crazier stocks, such as OTC and especially worthless penny stocks with market caps of $1 mil or less, what you say is a real problem (you can end up paying multiples of the last ticker if not careful) and you do have to be careful about it. Which is why you shouldn't trade penny stocks unless you know what you're doing (and if you're asking this question here, you don't)." "Why would people sell below the current price, and not within the range of the bid/ask? There are many scenarios where this is deliberate but all of them boil down to the fact that the top level's bid doesn't support the quantity you're trying to sell (or is otherwise bogus[1]). One scenario as an example: You're day-trading both sides but at the end of the day you accumulated a rather substantial long position in a stock.  You don't want to (or aren't allowed to[2]) be exposed overnight, however.  What do you do? You place an order that is highly likely to go through altogether.  There's several ways to achieve that but a very simple one is to look at the minimum bid level for which the bid side is willing to take all of your shares, then place a limit order for the total quantity at that price.  If your position doesn't fit into the top level bid that price will well be lower than the ""current"" bid. Footnotes: [1] Keyword: quote stuffing [2] Keyword: overnight margin (aka positional margin, as opposed to intraday margin), this is highly broker dependent, exchanges don't usually distinguish between intraday and overnight margins, instead they use the collective term maintenance margin" I can't say I know everything about the underlying details, but from what I understand, your limit buy adds to the bid side of open orders, and one possibility is that someone placed a market order to sell when the bid price for the stock fell to $10 which was matched to your open limit order. So using your terminology, I would say the spot bid price is what fell to $10, even if for a brief moment. Whether or not it is possible for your order to be filled when the limit buy price is deeper than the current bid price is beyond me. It may have something to do with lot sizes. "Most of the time* you're selling to other investors, not back to the company. The stock market is a collection of bid (buy offers) and asks (sell offers). When you sell your stock as a retail investor at the ""market"" price you're essentially just meeting whatever standing bid offers are on the market.  For very liquid stocks (e.g. Apple), you can pretty much always get the displayed price because so many stocks are being traded. However during periods of very high volatility or for low-volume stocks, the quoted price may not be indicative of what you actually pay.  As an example, let's say you have 5 stocks you're trying to sell and the bid-side order book is 2 stocks for $105, 2 for $100, and 5 for $95. In this scenario the quoted price will be $105 (the best bid price), but if you accept market price you'll settle 2 for 105, 2 for 100, and 1 for 95. After your sell order goes through, the new quoted price will be $95. For high volume stocks, there will usually be so many orders near the midpoint price ($105, in this case) that you won't see any price slippage for small orders.  You can also post limit orders, which are essentially open orders waiting to be filled like in the above example. They ensure you get the price you want, but you have no way to guarantee they'll be filled or not.  Edit: as a cool example, check out the bitcoin GDAX on coinbase for a live example of what the order book looks like for stocks. You'll see that the price of bitcoin will drift towards whichever direction has the less dense order book (e.g. price drifts upwards when there are far more bids than asks.)" Sounds like an illiquid option, if there are actually some bidders, market makers, then sell the option at market price (market sell order). If there are not market makers then place a really low limit sell order so that you can sit at the ask in the order book. A lot of time there is off-book liquidity, so there may be a party looking for buy liquidity. You can also exercise the option to book the loss (immediately selling the shares when they get delivered to you), if this is an American style option. But if the option is worthless then it is probably significantly underwater, and you'd end up losing a lot more as you'd buy the stock at the strike price but only be able to sell at its current market value. The loss could also be increased further if there are even MORE liquidity issues in the stock. If you are buying your order will be placed in Bid list. If you are selling your order will be placed in the Ask list. The highest Bid price will be placed at the top of the Bid list and the lowest Ask price will be placed at the top of the Ask list. When a Bid and Ask price are matched a transaction will take place and it will the last traded price. If you are looking to buy at a lower price, say $155.01, your Bid price will be placed 3rd in the Bid list, and unless the Ask prices fall to that level, your order will remain in the list until it trades, it expires or you cancel it. If prices don't fall to you Bid price you will not get a trade. If you wanted your trade to go through you could either place a limit buy order closer to the lowest Ask price (however this is still not a certainty), or to be certain place a market buy order which will trade at the lowest Ask price. "Alrighty.  So your question is, how confident are people that FB stock will close above a certain price at a certain time.  A ""call"" option contract allows you to buy shares in the future at a certain price.  FB calls are available for March of 2013.  The number in the ""strike"" column is the price you can buy FB stock at on the given date. Let's take the ""33.00"" strike.  This allows you to buy FB shares at $33 each next March.  If at that time it turns out they're worth $50 each, you can buy them at $33, then immediately sell them on the open market for $50, making money.  The person selling you the option knows that there's a chance the price will be above $33 in March, so he's going to charge you a few bucks to cover that possibility.  In this case, he's charging you $5.30 (as of me writing this).  So, you can have the option to buy FB shares at $33 each for $5.30.  This means the guy selling you the option is reasonably confident that in March, FB stock will be at or under $38.50, otherwise he'd lose money when you exercised the option.    Therefore, $38.50 is the option market's best guess as to the highest FB stock will be going for in March." "You can choose to place successively lower buy limit orders, but whether they get filled or not is not a given; it depends on whether sellers care to accept your bid. In your example of a 49.98 / 50.01 spread, if you place a buy with limit of 49.99, it won't get filled (if the order reaches the market while still at 49.98 / 50.01) immediately, but will be added to the order book.  By being added to the order book, the markets bid and ask become 49.99 / 50.01.  Your order won't get filled until some seller places a market order or a sell limit order of 49.99 or less.  No guarantee that that will happen, and even if it does, there's nothing to say that your follow-up buy at 49.98 will ever be filled.  In fact, your 49.98 buy order queues up at the ""end of the line"" behind all previously pending 49.98 bids, since your order arrived after those other bids.  Since the initial conditions you supposed had a 49.98 bid, such an order exists (or at least did exist; it might have been cancelled in the intervening moment. Basically, your first buy at 49.99, if it happens, has essentially no influence on whether your second buy at 49.98 will happen.  You can't expect to move the market lower by making a bid that is higher (49.99) than the existing best bid (49.98).  Whatever influence your 49.99 order has is to raise the market's price, not lower it."