Sending movers to come and take it back is insane. The most understanding I can be is that the seller just thought the OP would accept giving it back, and sent the movers out before contacting him. But even that's insanely presumptuous and still really rude.
But yeah, the OP made a deal and fulfilled his end of the deal by mailing the check and has already received the game. The transaction has been completed, whether or not the check has been cashed. It's your choice whether to go back on the deal, and it doesn't sound like you want to, which is completely fair.
Honestly, at this point, what you're protecting against is making your life harder later, if he tries to file a small claims case against you. If he doesn't cash your check, document attempts to pay him again and make sure that all future checks are sent with tracking and delivery receipt, to build a paper trail that you're acting in best effort (otherwise this can turn into a he said, she said sort of thing). Collect documentation on selling your two other games now, just in case (for what the lawyer mentioned earlier). I wouldn't go as far as hiring a lawyer, but I would collect a stack of documents I could easily give to one right away if it reaches that point.
edit: NM, it looks like this was resolved in the few minutes I was writing this up.