Without knowing when and how you paid for this and your reason for reurning it it's hard to advise you correctly.
eBay does not require sellers to accept returns unless the item is not as described, or the seller's policy is to accept returns - which will be stated in the lisiting.
If you were entitled to return it, it must be returned through eBay's appropriate procedure, according to the reason, see:
Returning an item for a refund: https://www.ebay.co.uk/help/buying/returns-refunds/return-item-refund?id=4041
If you return items outside of eBay's procedures it's between you and the seller and eBay won't get involved. If you were entitled to return it and are within the time limit you may still be able to open the appropriate return procedure through eBay and enter your postal tracking number to prove delivery.