Also you have to realise that nobody on the orderbook will pay you market rate. Bid orders will always be below market rate, meaning that if you buy from an ask order and sell to a bid order you'll always result in a loss even if the price stays the same or even goes up a bit.
This plus the 0.2% or so trading fees they take(which isn't much by crypto standards) is why you have less BTC than you began with.
Ah, I see, OP didn't understand how exchanges work!
Those differences come from other user's orders, not from exchange.
If on a certain exchange you can find people buying Bitcoin only at 1000$ now and you sell, the 7000$ profit is for the user who bought, not for the exchange. The exchange still gets only the displayed 0.20-0.25% fee and the withdrawal fees.
On Bittrex case don't forget about the withdrawal fees, since last time I've checked they were bigger than usual. (Now even withdrawals are tricky, I hope that you know that.)