Did you pay by paypal?
did you return by registered post or trackable to prove return?
If so open a dispute for item not as described, provide PP with the proof of return post when requested.
Advise the seller that feedback extortion is against ebay policy, that you are requesting a prompt refund or you will have no other choice than to forward their inappropriate demands to ebay.
If you paid by other means you are at the mercy of this seller, if the cost is warranted then you might have to cave to their demands, and hope they refund.