why would someone buy or sell just a few shares in stocks
Simple, there is no magic price adjustment after sales - why do you expect the stock price to change? The listed price of a stock is what someone was willing to pay for it in the last deal that was concluded. If any amount of stock changes ownership, this might have the effect that other people are willing to buy it for a higher price - or not. It is solely in the next buyer's decision what he is willing to pay. Example: if you think Apple stocks are worth 500$ a piece, and I buy a million of them, you might still think they are worth 500$. Or you might see this as a reason that they are worth 505$ now. I buy or sell a few shares whenever I have a small amount of money to transfer into out of my mutual funds. Since I'm not paying transaction fees, there is no reason not to, when that is what makes sense. If you are paying a fee for each transaction, of course it makes sense to try to wait until you have a larger amount to move so the overhead per share is lower.
"Usually, the amswer to ""why sell it"" is ""to maintain the specific distribution balance, or to track the index, that this fund was designed to offer."" A ""buy and hold"" fund could only buy when users are actively putting money into it. That limits their ability to follow those approaches.  And I think there would be problems msking withdrawls/redeptions ""fair"", in terms of what shares are sold and how the costs for selling them are distributed, that don't arise for a single buy-and-hold investor. If you're willing to accept the limitations of the former, and can overcome the latter, it's an interesting idea...  But note that one of the places index funds save money is that, since the composition of indexes changes rately, they are already operating mostly in buy-and-hold mode.It's unclear how much your variant would save. Worth exploring in greater depth, though. I think." So in your screenshot, someone or some group of someones is willing to buy 3,000 shares at $3.45, and someone or some group of someones is willing to sell 2,000 shares at 3.88. Without getting in to the specific mechanics, you can place a market buy order for 10 (or whatever number) shares and it will probably transact at $3.88 per share because that's the lowest price for which someone will currently sell their shares. As a small fish, you can generally ignore the volume notations in the bid/ask quotes. "The reason for selling a stock ""short"", is for when you believe the stock value will decrease in the near future. Here is an example: Today Exxon-Mobile stock is selling for $100 / share. You are expecting the price to decrease, so you want to short the stock, which means your broker (i.e. eTrade, etc) allows you to borrow shares without paying money, and those shares are transferred into your account, and then you sell them and receive money for the sale. But you didn't actually own those shares, you only borrowed them, so you need to return the shares to your broker sometime in the future.  Let's say you borrow 10 shares @ $100, and you sell them at the market price of $100, you receive $1,000 in your account. But you owe your broker 10 shares, which you need to return sometime in the future. A few days later, the share price has decreased to $80. Now you can buy 10 shares from the market at a total cost of $800. You get 10 shares, and return those shares to your broker. Since you originally took in $1,000, and you just paid out $800, you keep a resulting profit of $200" "First a quick terminology correction: I believe you're proposing selling 10,000 shares of the stock of a company, not ""10,000 stocks"". When you sell, you need to decide whether you're selling for a specific minimum price or just selling for whatever price you can get. If you set a specific lower limit on asking price, then if people aren't interested at that price it doesn't sell. Which may mean you sell only a few shares, or none if your asking price isn't considered reasonable. If you want to sell independent of price, then as you begin to flood the market with your shares, the price you get per additional share may decline until it finds a buyer. What that lower limit is will depend on what people think the stock is currently worth. This is one of the many complications I don't want to deal with, which is why I stick with index funds." The main reason I'm aware of that very few individuals do this sort of trading is that you're not taking into account the transaction costs, which can and will be considerable for a small-time investor. Say your transaction costs you $12, that means in order to come out ahead you'll have to have a fairly large position in a given instrument to make that fee back and some money. Most smaller investors wouldn't really want to tie up 5-6 figures for a day on the chance that you'll get $100 back. The economics change for investment firms, especially market makers that get special low fees for being a market maker (ie, offering liquidity by quoting all the time). "In fact, it's quite the opposite. If someone is willing to sell some stock as low as $30/share, and someone else is willing to pay $31/share, one of those individuals is going to get a good deal - unless HFT acts as the middle-man and snags the extra dollar. In which case the individuals get the worst price they would accept and someone with fast collocated computers gets to skim some profits (while adding no value, the order could have happened without the so called ""liquidity"" that HFT claims to add).  It's not exactly that simple, but that's the basic effect. It's an unnecessary middleman that skims profits away from individuals on both sides.  I would like to see a return to ""investing"" back to the meaning of the word, instead of gambling on daily or short-term fluctuations. I wouldn't mind a long-term holding requirement (3 months?) for every purchase, and a daily exchange-calculated set price (calculated by actual orders placed) that everyone who buys/sells a particular stock on a given day pays.  Yes, my ideas would destroy an entire industry. I'm ok with that because it would encourage people to *really* invest in companies." A stock is only worth what someone is willing to pay for it. If it trades different values on different days, that means someone was willing to pay a higher price OR someone was willing to sell at a lower price.  There is no rule to prevent a stock from trading at $10 and then $100 the very next trade... or $1 the very next trade. (Though exchanges or regulators may halt trading, cancel trades, or impose limits on large price movements as they deem necessary, but this is beside the point I'm trying to illustrate). Asking what happens from the close of one day to the open of the next is like asking what happens from one trade to the next trade... someone simply decided to sell or pay a different price.  Nothing needs to have happened in between. As far as i understand the big companies on the stock markets have automated processes that sit VERY close to the stock feeds and continually processes these with the intention of identifying an opportunity to take multiple small lots and buy/sell them as a big lot or vice/versa and do this before a buy or sell completes, thus enabling them to intercept the trade and make a small profit on the delta. With enough of these small gains on enough shares they make big profits and with near zero chance of losing. Insiders are prevented from buying or selling shares except at certain periods right after information is disclosed publicly. But. People have bills to pay and kids to put through college and whatnot. So an insider can set up a plan where shares are sold on a specific schedule and they have no control over number of shares or timing. These plans (covered under rule 10b5-1) allow insiders to generate cash flow without immoderately benefiting from their inside information. Sales under these plans can mostly be ignored when trying to figure out the fortunes of a company from insider trades. "As the other answer said, the person who owns the lent stock does not benefit directly.  They may benefit indirectly in that brokers can use the short lending profits to reduce their fees or in that they have the option to short other stocks at the same terms.   Follow-up question: what prevents the broker lending the shares for a very short time (less than a day), pocketing the interest and returning the lenders their shares without much change in share price (because borrowing period was very short). What prevents them from doing that many times a day ? Lack of market.   Short selling for short periods of time isn't so common as to allow for ""many"" times a day.  Some day traders may do it occasionally, but I don't know that it would be a reliable business model to supply them.  If there are enough people interested in shorting the stock, they will probably want to hold onto it long enough for the anticipated movement to happen.   There are transaction costs here.  Both fees for trading at all and the extra charges for short sale borrowing and interest.  Most stocks do not move down by large enough amounts ""many"" times a day.  Their fluctuations are smaller.  If the stock doesn't move enough to cover the transaction fees, then that seller lost money overall.  Over time, sellers like that will stop trading, as they will lose all their money.   All that said, there are no legal blocks to loaning the stock out many times, just practical ones.  If a stock was varying wildly for some bizarre reason, it could happen." "As @ApplePie pointed out in their answer, at any given time there is a finite amount of stock available in a company. One subtlety you may be missing is that there is always a price associated with an offer to buy shares. That is, you don't put in an order simply to buy 1 share of ABC, you put in an order to buy 1 share of ABC for $10. If no one is willing to sell a share of ABC for $10, then your order will go unfilled. This happens millions of times a day as traders try to figure the cheapest price they can get for a stock. Practically speaking, there is always a price at which people are willing to sell their shares. You can put in a market order for 1 share of ABC, which says essentially ""I want one share of ABC, and I will pay whatever the market deems to be the price"". Your broker will find you 1 share, but you may be very unhappy about the price you have to pay! While it's very rare for a market to have nobody willing to sell at any price, it occasionally happens that no one is willing to buy at any price. This causes a market crash, as in the 2007-2008 financial crisis, when suddenly everyone became very suspicious of how much debt the major banks actually held, and for a few days, very few traders were willing to buy bank stocks at any price." Because people bought their shares under the premise that they would make more money and if the company completely lied about that they will be subject to several civil and criminal violations. If people didn't believe the company was going to make more money, they would have valued their shares lower during the IPO by not forming much of a market at all. Consider the case where a stock has low volume.  If the stock normally has a few hundred shares trade each minute and you want to buy 10,000 shares then chances are you'll move the market by driving up the price to find enough sellers so that you can get all those shares.  Similarly, if you sell way more than the typical volume, this can be an issue. For the same reason you wanted it when you bought it. No-one guarantees that you'll be able to sell the stock you hold, and in fact many people get stuck with stocks they'd like to sell, but no-one is buying. But if investors think there's a profit potential that is not exhausted yet - they'll want to buy the stock. Because they receive compensation (generally interest + dividends) for loaning out the shares. I own an asset X.   Somebody else wants to borrow asset X for some time period.   I agree to loan them asset X in return for some form of compensation (generally a rate of interest plus, in this specific case, any dividend payments).  The reasons why I own asset X, and why they want to borrow asset X are irrelevant to the transaction. The only relevant points are the amount of compensation and the risk that they might default on the loan. This applies equally well to shares as to money or any other kind of loan-able asset.