So you think or say that maybe doing inside or background trading for Coins that they have on there Exchange ?

If you doing that it would be the best to move that thread into  Bitcoin Forum > Economy > Trading Discussion > Scam Accusations section !
You can do it by yourself on the left bottom corner you will see move topic click on it and move it to the section.

Do you have any proof that they doing maybe ?
Also its possible that they have trading bots and after you placed your sell and buy order they stoped because it was to high for them.