Ebay's Money Back Guarantee, along with PayPal's Buyer Protection, can legally only cover items which you can see are damaged, faulty, or not as described on receipt. So items must be returned in the exact condition in which they were received. Ebay and PayPal (or any payment processor) cannot possibly assist with items which fail within warranty, as they can have no idea who is at fault - you for misusing the item, or the seller for supplying substandard goods. Only a judge can make such a decision.
But Ebay's MBG and PayPal's Buyer Protection are generous extras to your consumer rights - they don't replace them. If the seller is a UK business seller and continues to be unhelpful, go to the CAB's website to learn what to do ('before action' letter by Signed For post, followed by Small Claims court if necessary) just as you would when dealing with an unhelpful retailer in your local mall.
If the seller is not in the UK, there is nothing you can do, I'm afraid.
