You need to pay the return shipping charges, and the seller has no obligation to refund you your initial shipping cost either, since the return was entirely your own issue. That said, they can't accept the return, and then not give you a refund. The correct course of action for the seller would have been to list the item with no returns accepted, then refuse your return, citing that they only accept returns for items that arrive not as described. In those situations, shipping is covered by the seller.
I would suggest you explain the above to them, and tell them to take the shipping charges out of your refund; eBay has a provision for this, but I believe for it to be done automatically, they had to choose the correct option when they sent you the label.
Still, they are being a dick, but you might as well be the bigger person and follow the rules.