I agree, I have sold several used vehicles and use a popular legal guide as a template for a Buyer's Order which should be what you had this person sign.

It states something to the fact that I do not warranty the product or provide any warranty other than what the original manufacturer provided when sold new. You should always keep a copy of this document in case you are sued which you are being now. It will be your word against the buyers and who knows which the judge will believe. Good Luck with it.