Sellers can only refuse of change of mind returns.
They have to accept not as discribed cases.
The seller should pay for the return and supply a tracked label.
As long as tracking shows attempted delivery back to the seller you should get a refund with 48 hours ( I think).
If the seller doesn't give you a label you escalate the case like eBay told you.
A button will appear on the case when you can ask eBay to step in.
Also if they don't refund within 48 hours you should contact eBay CS who should help you.
Here's a link to contact customer services incase you need it.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
Also this page will explain about return cases.
https://www.ebay.co.uk/help/buying/returns-refunds/return-item-refund?id=4041