I guess it depends on what kind of expectations you set with the buyer. Do they think they are getting a game with bulletproofed boards or do they think they are getting a game with working boards. Maybe give them 30 days to decide if everything is fine. Or just tell them straight up once it leaves, you own it and any potential problems.
I think if you advertise the game as working and you sell many games you should help people out for at least a short period of time, but set a timetable to it.