eBay's money back guarantee applies in the same way to large corporate sellers as to small sellers.

So does the 30 day time limit to open a case. From what you have told us it sounds as though you may not have opened one? If not, it will be too late now.

As a bank chargeback isn't available in this case you may just have to resolve this with the seller yourself. It's difficult to offer suggestions as we don't know what happenned. You could speak with Citizens Advice, or if there's enough money involved consult a solicitor.