EBay's money back guarantee covers most items sold here, but only for 30 days after delivery. After that, they will not get involved, whatever the problem. PayPal buyer protection lasts longer, for 180 days, but only covers faults present on delivery. Neither will get involved in warranties or faults that occur in use. If you paid through another payment provider you can check their terms to see whether they provide any protection.
Otherwise, any responsibility now rests with the seller. If they're a UK business seller, you have your statutory consumer rights. Unfortunately, having rights means nothing unless you can enforce them, which can be difficult or impossible for items bought on eBay. Even manufacturers' warranties are liable to be invalid for items bought here.
You can consult trading standards or citizens advice, or consider using the small claims court or the Courts and Tribunals internet-based money claim service: https://www.moneyclaim.gov.uk/web/mcol/welcome.
When buying from private sellers or sellers in other countries, there's unlikely to be anything you can do.
These are the risks of buying from unknown sellers here. EBay is not like the shops.
