Part of buying a used anything is not only the wear-and-tear, but the risk when there's no warranty. None of us know what was said between you and the buyer. But unless you made some type of warranty, and if to the best of your knowledge the bike was sound when you sold it and/or you didn't withhold any adverse information that could be related to the issue he's had, then you have no obligation to compensate him for anything. That's not to say, if your conscience is at issue and you feel an obligation, you can't work something out with him, based upon your understanding of the situation. (I wouldn't, but that's because I understand some people will lie without qualms if they think they can get something for themselves out of it. Truth is secondary to these types, if even that.) Negotiations take place BEFORE a deal is closed. Not after. He's trying to re-negotiate. I'd be polite, and understanding. But I'd tell him, nicely, to 'pack sand'. Next time you sell something, if you didn't this time, make it absolutely clear to the buyer that they need to do all discovery, research, etc. before they hand you the money, and you sign the vehicle over.