i know theres a lot of things you CANT check when it comes to an inside of a motor... and usually sales are branded "AS-IS"... you gotta, GOTTA know what you're buying before you exchange money. once that money switches hands its a done deal and whatever happens happens... usually go for a test or at least run it on a trailer. do compression check, etc.
worst case you could take him to small claims over it, but most likely you're gonna lose.