It's important to understand that eBay's money back guarantee only lasts for 30 days. After that, eBay won't assist you, whatever happens. Paypal's buyer protection policy lasts longer, for 180 days, but only covers non-delivery or the condition of items on receipt, not faults that occur in use. Neither will get involved in warranties.
Any responsibility now rests with the seller. If they're a UK business seller you should, in theory, have the same statutory consumer rights as if you bought it from a shop.
In practice, having consumer rights means nothing unless you can enforce them, which is often difficult or impossible against distant eBay sellers. Even manufacturers' warranties are liable to be invalid for items bought here.
If no success with the seller, you can consult trading standards or citizens advice or, if worthwhile, consider using the small claims court or the Courts and Tribunals internet based money claim service, see: https://www.moneyclaim.gov.uk/web/mcol/welcome.
It the seller is a private seller, or abroad, there's nothing you can do.
Good luck, but buying from distant, unknown sellers on eBay isn't like the shops.
