How have you gone about resolving your problem? All disputes must be resolved through the Resolution Centre.
http://resolutioncentre.eBay.co.uk/
Have you opened a case? If not do so now.
Did you use fully tracked postage?
As you have already returned the item you should put the tracking into the case details to prove you sent the item back.
Unless you can prove to eBay that you sent the item back by adding tracking to the case, eBay will not force the seller to refund you.
