As long as you described the game the best you possibly could, and the buyer gave you the thumbs up to buy it, once he leaves you don't have any kind of obligation to him anymore. Now what makes a good seller is following up and making sure they are happy though. I mean, you shouldn't have to bend over backwards for them if something goes wrong, but you can certainly give them some helpful advice and even point them in the right direction. Usually point a to point b problems are simple fixes. Once someone decides to threaten you in any way, then they can just screw off though. If the guy is nice then I definitely have no problem working with him should a problem arise.