I think you need some legal advice here. I have no idea which contract takes precedence.
Did the eBay listing have any small print about time taken to pay deposit which means the sellers contract with the eBay buyer is void? If so, then the car is legally yours. If not though, he has an earlier contract, so possibly has more claim to it than you.
I wouldn't be too harsh in your dealings with the seller yet - it seems at face value that he has done everything in good faith, it's just things have gone pear shaped.
Good luck!
"Human beings, who are almost unique in having the ability to learn from the experience of others, are also remarkable for their apparent disinclination to do so." - Douglas Adams
I
Nomes &
Gibbo