Well, do you want the bike? I'd go check it out, and if it isn't up to snuff, you just say, "no thanks, this isn't quite what I'm looking for."

No harm no foul, you haven't BOUGHT the bike yet. No obligato. Caveat Emptor.

Bring a chain checker, psych the guy out. worn chain, poss worn drive train, poss new rings cass, chain. rims true?

anything you don't like about it can be your reason not to buy.