Always sucks when this happens! However, theres no saying that the piston didn't crack when you were driving home from buying it... I once sold a CD player on eBay, in fine working condition. 2 weeks after I'd sent it, the new owner opened a case on me saying that it didn't work... He won the case through ebay because the customer always wins lol. He sent me back a broken CD player and I lost £104 which I'm still waiting to recieve from eBay after they "reviewed the case" and said I was not to blame after sending numerous pictures of it working and boxed up etc... The fact its 2 weeks later raises a lot of questions. For example, the CD player I sent just happened to be "sat in the box for 2 weeks before he took it out and tried it"........ Anything could have happened in those 2 weeks but unfortunately because you did the sale privately, it pretty much is "Sold as seen"... Like everyone else has said, never buy a car if its warm when you view it. Guess I can't offer any advice other than get it fixed, and have some fun in it lol!