If you missed the time limit for eBay's money back guarantee, and didn't pay by PayPal you have a problem.
At one time, if you paid through eBay's checkout by a bank credit or debit card the payment was processed through PayPal, meaning that you could open a PayPal account and use PayPal's buyer protection policy. I don't know whether this still applies under eBay's new managed payments system, but I doubt it. Someone here may know?
Otherwise you could ask your bank about the possibility of issuing a chargeback. Unfortunately this is uncertain, as the payment is not made to the seller, but to eBay. For the same reason, card payments on eBay are probably not eligible for statutory consumer credit act protection, as they would be if the card payment was made to the seller direct.
Otherwise, you can consult trading standards or citizens advice or, if there's enough money involved and you can prove your case, consider using the small claims court or the Courts and Tribunals internet-based money claim service: https://www.moneyclaim.gov.uk/web/mcol/welcome
