I think that if its clear its a trade sale, and importantly you don't give a description (other than factual one i.e. make, model, spec etc) or discuss if its fit for his purpose, then this potentially only leaves "of satisfactory quality" which is a relative term related to age, mileage and price. If it went to a small claims court the buyer would somehow need to satisfy this test.
If the buyer is the type who wants to go all the way i'd just give him a refund and sell it to someone else. Hopefully he might realise his case isn't as watertight as he may have thought and take it on the chin.