If he's paid by paypal it will be on the transaction details. Youll be able to get a contact number if you request it from ebay anyway.

Personally, i wouldnt go selling it again and keep his money as thats just morally wrong in my opinion. If you still get no joy, id just stick it in the loft and forget about it. Provided he's still with us, im sure he'll contact you at some stage.