If you stuck to the rules then this could not happen. DIGITAL ITEMS cannot be sold, if the buyer claims not received and opens a case under the Money Back Guarantee, you haven't a leg to stand on. You will have to refund or if seller escalates, ebay will force a refund.
You must read PROHIBITED ITEMS, and should have considered that it is risky selling something that you cannot prove by tracking that buyer has received it.
You can hardly complain to ebay that buyer is trying it on as you were wrong to sell in the first place.
