If you don't tell us what you've already done it's hard for us to suggest what you should do.

Why didn't you cancel the sale through eBay while you still could, to get your selling fees returned? What is the value of the car (a banger or a high end vehicle?) Have you tried to contact the buyer, with what result? Have you written to them with a deadline for collection? Have you taken legal advice, or consulted citizens advice?

If it's a banger the quick and easy answer could be just to refund the origInal payment. If there's money involved handing it back it may be more complicated. There could even be risks involved if their original payment was made from fraudulent funds - you could have been set up for a scam.

Take proper advice.