Depends...if he takes you to small claims court, he could win. The judge can rule whatever they want without evidence in either direction. Now, more than likely, you would win because as others have stated "Buyer beware". If you sold an ATV with a known major issue then that is kind of a scumbag thing to do. If you sold it with a known issue and told the buyer about the issue, then that is definitely on them. If you sold it with no known issues and it suddenly developed one...that is also on them.