If the account was in fact hacked, then Quickseller, knowingly or otherwise sold stolen property. If he never had legal ownership of it to begin with, he has no right to keep the collected funds because there was no valid exchange. The rest of the argument is irrelevant.

Just a quick analogy for you for comparison...
http://nccriminallaw.sog.unc.edu/must-a-pawn-shop-return-stolen-property-to-its-owner/

He may or may not have known it was hacked, but as far as possession and the law are concerned it doesn't make any difference, he must return the property and the funds, and be more selective about accounts he buys in the future.