Nothing can be done via eBay, their 30 Day Money Back Guarantee has long passed.
Any warranty offered by a seller is only ever going to be as good as your ability to enforce it and, as you've discovered, you can't.
Is your seller a UK business? If so, contact the Consumer Action Group (the first site that comes up on Google) and ask there if there's a route that you can follow to get a refund here. If you paid with a debit or credit card it should be easy enough to issue a chargeback.