I presume this was a cash purchase?
There is no finance?
And he still has the car to sell?
The starting point is that you are in breach of contract unless you agreed cancellation rights.
However, he only has a claim for the sum of his loss and he still has the car to sell. Therefore he hasn't provided consideration for the contract.
He may have a claim for a small amount of admin costs and maybe some readvertising costs but that would be a very low sum and certainly nothing like the sum you have paid.
If he won't agree to refund you then just sue him at the small claims court although I would warn him first before you actually issue against him.
Can I clarify anything for you?