Hi,
I just had a thought that sounds pretty obvious, but maybe there is an obvious answer which I don't see.

So, imagine such a situation:
The best sell BTC order on an exchange site is 10USD/BTC. I put in a buy BTC @11USD/BTC, and am counting on it to be fulfilled at 10USD/BTC, because there is such a sell offer.
Now, the exchange sees those two offers, and instead of matching them, they buy from the other guy at 10USD/BTC, and sell me for 10.8USD/BTC. And then they say: a dark pool buy order came in at 10.2USD/BTC, and we matched the 10USD/BTC sell order with this, while your buy @11USD/BTC order got fulfilled by a dark pool sell order at 10.8USD/BTC.
Is this possible, or am I missing something? And if it is, then how do you know if this is not happening all the time?
Thanks,
Bitboy999
and then mtgox/tradehill instead of making the sale at $10 places it's own Buy@10.1, gets the bitcoins and then does a Sell@10.8. Meaning that the original buyer pays 0.8 BTC more than they should, and the exchange owner makes $0.7 extra profit free?
If this is the case it seems like people place a lot of trust in the exchange owners....