Well first of all I suggest you contact PayPal and eBay and explain the situation. Also contact the buyer and ask in detail why he or she is unsatisfied with the product which was sold to he/she.
I've been in this situation before when I sold a Nintendo Wii on eBay, someone came and picked it up, didn't think it was up to high standards (even though it was new and never opened) and filed a PayPal Claim against me.
Just be nice to the buyer and don't get angry as this wont help you in anyway. Negotiate with the buyer and ask he or she to return the car and once that's done, you can go ahead and give the buyer the refund.
If the buyer doesn't return the car I'd have a talk with PayPal (on the phone and not by email!) eBay and get the police involved.
Hope this helps.
