I see, so if a bank robber buys a car from you and you make a nice little profit on that car, you should give it back if the robber is caught? The same with the restaurant owner, the grocer, the utility company etc. After all, they profited from his illegal actions as well.
Except that the government had the expressed responsibility for monitoring the legality of the situation. If a thief brings stolen goods to to a buyer, the buyer is in fact responsible for attempting to verify the legality of the source. This situation is more akin the police being payed to look the other way.