When a buyer opens a not received case and there is no tracking proving delivery, then ebay refund NOT YOU THE SELLER.
If there was no tracking in the first place then you would have no proof of even sending their purchase, and because you had not followed the system you as seller would have to refund.
You mention proof of buyer receiving their purchase! What proof? If you really do have tracked proof which was loaded onto ebay, then appeal ebays decision to refund.