Sounds like it was stolen in transit. Ask the buyer to open a return. You should get the packaging back to support your claim from the courier. Once you have it back issue a refund to the buyer and claim for yourself.
If the item was relatively cheap you might decide it's not worth paying the return postage to get the packaging back. You can just refund without return. It's up to you.
If you bought the postage through ebay here's how to claim:
https://support.packlink.com/hc/en-gb/articles/5528489675922-How-can-I-request-compensation-for-loss....
If you bought from the courier including Royal Mail you claim direct from them via their website.