Did the buyer open a case for item not received and you refunded through those case details, then you can claim now from Royal Mail ( if they were used, or claim from courier used) for compensation for item lost, using your proof of posting.
You cannot claim anything from eBay especially if you refunded outside a case.
One more thing, if you used Simple Delivery for this item, then all the buyer had to do for their refund, was to open a case, and eBay would have taken over and paid the refund, and you keep the payment from the sale.
@redwood_001