Thanks for all your responses. He brought it back and I took a close look at it and could see no sign of abuse. Audio Research is notorious for having the toggle on/off switch go out through constant use. I'm betting that it was just bad luck and it happened after he took it home. He still wanted the unit, so, we agreed that I would keep half the money until it was repaired, which I would bear the responsibility for, and then he would return to pick it up.
That seemed like a fair compromise. This way if the repairs show abuse I use the money he left to pay for it and return the rest making him responsible. If the dealer sees it as a non-abuse repair situation I pay for all the repair and still sell him the pre-amp. I think in dealing with used equipment you always should treat the other party the way you would want to be treated, even if they are obnoxious.
That seemed like a fair compromise. This way if the repairs show abuse I use the money he left to pay for it and return the rest making him responsible. If the dealer sees it as a non-abuse repair situation I pay for all the repair and still sell him the pre-amp. I think in dealing with used equipment you always should treat the other party the way you would want to be treated, even if they are obnoxious.