Have you checked on Royal Mails tracking page as sometimes they show the GPS delivery location on the map?
If it's there you may be able to work out who has it, or use it as proof it's not been delivered correctly..
You could also ask your postie where they delivered it to, if not much time has passed they may remember.
This would be the quickest way to solve it, if it's possible.
Failing that you could try to appeal Ebay's decision - you have 30 days to appeal from the date the case closed so you have some time to see if you can find it or get something from Royal Mail or see if the seller helps any further:
https://www.ebay.co.uk/help/buying/returns-items-not-received-refunds-buyers/appeal-outcome-case-buy...
To add... I would also send a complaint to Royal Mail if the postie is no help.
“We haven’t got a plan so nothing can go wrong!” Spike Milligan