Possibilities, not necessarily with your situation:
¶ the buyer could say/lie that the parcel was received, but the package was empty or contained a brick, and would get refunded of the seller's money by eBay/PayPal.
¶ it is not unheard of that the postman scans for delivery at the post office, then delivers it later but loses it along the way.
¶ postman cound scan parcel as delivered, and parcel could be left at door or by mail box and stolen, even stolen from mail box.
¶ insurance could offer some succor in these case.
Robert