i wouldn't worry about it - if he has taken three weeks to contact you, then that's his problem.
check his ebay activity and see if he has bought anything related to the car/engine and if he has use that proof of use if/when he starts a paypal claim. if the worst comes to the worst paypal will refund him and you will have your engine back - infact when the paypal claim is in, ask to see the engine prior to any claim being considered.
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