This question is asked so often, and the answer is not very good news.
In theory, this is covered by eBay's condition of returned items: https://www.ebay.co.uk/help/policies/member-behaviour-policies/condition-returned-items-policy?id=47...
In practice, you may be lucky, as some sellers have been, but others have told us that eBay's response to items being returned damaged or swapped for another one was that as eBay didn't see the item, either as received by the buyer or as returned to the seller, they are not in a position to verify who has the valid complaint, or "take sides". EBay's idea of not taking sides is to apply their defect policy for cases of uncertainty, which is to find in the buyers favour and require the seller to refund them.
I suspect that one reason for this policy is that if eBay turned down a case that was genuine, the buyer could sue them for failing to honour their money back guarantee. EBay doesn't give any guarantees to sellers, so the safe and easy answer is to make the seller refund them.
If necessary, you can appeal eBay's decision when the item is returned, just don't build too many hopes succeeding: https://www.ebay.co.uk/help/selling/managing-returns-refunds/appeal-ebays-decision-return-missing-it...
EBay's user agreement requires us to accept their decisions. As eBay is unregulated there is no right to any independent appeal. Good luck anyway!
