oh right. In which case, i would presume that the fulfilment of the contract is when the car is delivered to you - after all, if it was when you paid, then he could very easily just not send you the car but still say "i fulfillled my part of the contract by accepting payment" I have no basis in law for this, but it's what common sense is telling me.
i would have thought that you would have a right to inspect the car, and then accept or reject it immediately (so for example, you couldn't drive it for 3 weeks and then reject it wth no grounds) However, if you are rejecting it for no reason i would havethought you would have to bear the delivery costs..
All of the above is just what i would think though, not sure if it's right.
i had a friend who bought a van on ebay, paid, went to pick it up and it broke down about a mile away from the persons house. He immediately went back to them and rejected the vehicle because of this, but they refused to refund his money. The friend took him to court, but lost.