It's why I never take more than a couple of hundred in deposit, £500 notes is a good chunk and worth kicking off about, £200 is a reasonable amount to have to re-advertise etc.

Personally I would tell him he can bring the parts back and you will give him £300 and the rest you will be using to re-advertise, not forgetting that now it looks suspect to other potenial buyers that you told the car was sold, that the sale fell through.