It seems rather simple;

Hacker gained access to OP's account.

Hacker cleansed account of BTC address posts / changed email / password.

Hacker contacted Quickseller and acted as though he was OP.

Quickseller thought he was dealing with OP.

Quickseller sold account to other person.


If it were me that was making the decision, OP would get his account back. Quickseller refunds the buyer his amount. Quickseller takes a loss on the account. Quickseller should verify account ownership better.