Are you returning them in the same condition as received? That is if they were brand new in packaging was the packaging opened? If so, the seller can offer a partial refund.
Have you opened a return case and the partial offer is in the case or is this just through eBay messages?
If accepted by seller, a business seller has to accept, you as buyer have to pay return postage and should receive 100% refund, just as if you had returned to M & S.
