Sadly this is such a common story. eBay's own money back guarantee only lasts for 30 days, after which it's between you and the seller. And experience has shown that for items bought on eBay warranties often prove to be worthless, ans sellers don't want to know.
Depending upon how and when you paid, your payment provider e.g. credit or debit card, Google or Apple Pay may be able to help. The time limit is usually around 120 days, but this may vary.
Failing this, you can consult citizen's advice about other options. If the seller is a UK registered business seller you should still have some statutory consumer rights, but in reality having rights doesn't count for much unless you are able to enforce them. Which can range from being difficult to utterly pointless in the case of distant, uncooperarive eBay sellers. Good luck anyway.