user8997: I want to understand something very basic. How does trading work between BTC & USD. 
Let's say I have 1 BTC at US 100. The naive idea is "buy low" and "sell high". But I don't grasp how that work, ie, how is possible to profit in both directions.
If the value of BTC change this way (value in USD):

110 85 102 40 70 90 115 130 100 200

What I need to do to turn a profit? And if my plan is that I don't lose more than I invest at the start?

user7453: Pretty simple. You just make money on the differences when you buy/sell (taking in to account fees).  
In your example, you start with 1 BTC purchased at $100 USD.  Assuming no fees, you would:

Sell at $110 (Profit: $10)
Buy at $85
Sell at $102 (Profit: $17, total: $27)
... and  so forth

Now, you could get even more creative, since you can purchase fractions of BTC.  In this case, you could:

Sell at $110 (Profit $10)
Buy 1.294 BTC at $85 each for $109.99
Sell at $102 for $131.98 (Total Profit: $31.98)
...


8997: <p>I want to understand something very basic. How does trading work between BTC &amp; USD. </p>

<p>Let's say I have 1 BTC at US 100. The naive idea is "buy low" and "sell high". But I don't grasp how that work, ie, how is possible to profit in both directions.</p>

<p>If the value of BTC change this way (value in USD):</p>

<blockquote>
  <p>110 85 102 40 70 90 115 130 100 200</p>
</blockquote>

<p>What I need to do to turn a profit? And if my plan is that I don't lose more than I invest at the start?</p>
 7453: <p>Pretty simple. You just make money on the differences when you buy/sell (taking in to account fees).  </p>

<p>In your example, you start with 1 BTC purchased at $100 USD.  Assuming no fees, you would:</p>

<ul>
<li>Sell at $110 (Profit: $10)</li>
<li>Buy at $85</li>
<li>Sell at $102 (Profit: $17, total: $27)</li>
<li>... and  so forth</li>
</ul>

<p>Now, you could get even more creative, since you can purchase fractions of BTC.  In this case, you could:</p>

<ul>
<li>Sell at $110 (Profit $10)</li>
<li>Buy 1.294 BTC at $85 each for $109.99</li>
<li>Sell at $102 for $131.98 (Total Profit: $31.98)</li>
<li>...</li>
</ul>
