You can't return it through eBay, unless the fault occurred within eBay's own 30 day money back guarantee period. for most items sold here, but after that they will have no further interest.
If it's still under the manufacturer's gaurantee you can try using this, but manufacturers warranties are often invalid for items bought on eBay.
Otherwise, any responsibility now rests with the seller. If they're a UK business seller you may have some statutory consumer rights, but enforcing them for items bought on eBay can be difficult or impossible. You can ask ciizens advice or trading standards for advice.
These are some of the risks of buying on eBay - it isn't like the shops.
