user-1: I have small question, where does bitcoin "banks" keep money you pay them for bitcoins? do they keep it in some kinda bank ?
and when we sell bitcoins to dollars what fee do we pay?

user-1: Bitcoin "banks" are markets, not savings institutions. When you buy a bitcoin, you have bought a bitcoin; the money becomes the seller's. When you sell a bitcoin the money becomes yours and the bitcoin becomes theirs; similarly when you buy something with a bitcoin. 
Since the value of bitcoins is not nailed down, there's no question of a transaction fee; you just agree with the other person on what the price of the goods is in bitcoins (or on what the conversion rate to/from other currencies will be). If you don't like the price or rate one person charges, you go elsewhere.

-1: <p>I have small question, where does bitcoin "banks" keep money you pay them for bitcoins? do they keep it in some kinda bank ?</p>

<p>and when we sell bitcoins to dollars what fee do we pay?</p>
 -1: <p>Bitcoin "banks" are markets, not savings institutions. When you buy a bitcoin, you have <em>bought</em> a bitcoin; the money becomes the seller's. When you sell a bitcoin the money becomes yours and the bitcoin becomes theirs; similarly when you buy something with a bitcoin. </p>

<p>Since the value of bitcoins is not nailed down, there's no question of a transaction fee; you just agree with the other person on what the price of the goods is in bitcoins (or on what the conversion rate to/from other currencies will be). If you don't like the price or rate one person charges, you go elsewhere.</p>
