It would have helped to know whether the listing stated about your seller's returns policy. If they don't accept cancelaltions, it was their right to refuse your request. Once a request has been made and refused, a second request can't be made. If they have changed their mind it's between the two of you what to do now. It will be too late for the seller to have his selling fee returned.
If the seller states that they accept returns but has refused to do so, this would be a very different situation. I think you can still open a money back guarantee case after a cancellation has been closed. EBay's MBG policy states that if a seller won't honour their returns policy, the case can be treated as though the item was not as described. This is quite unusual, so if in any doubt about the procedure ask eBay customer support.
