ORIGINAL: carrellh
If you used Paypal;
File a dispute with them asking for a refund
Send the engine back at your expense
They will almost always force the seller to refund if the buyer sends the item back at his own expense
I would guess you best use registered mail or courier or something, but the guy can refuse it if hew recognizes the sender can't he? How do you ensure the seller cannot claim he never got it?