If the guy will let you I would try to apply pressure to the system, if you dont have one you can ent one from an autozone, and see if it holds pressure. If not and you cant see where its coming from it could be the headgasket and run away unless youre getting an awesome deal, but if it holds pressure it could be a simple fix like a tstat or bleeding.

 

I'd definetely see about having the problem fixed before you buy one big headache.