Does the tracking show that the item was delivered to the seller?
I am not insulting your intelligence, but are you looking at the right number on the Post Office proof of posting?
Usually, the tracking is added automatically to the case and then the item is shown as sent when it is scanned at the Post Office.
Go back to the case and see what the current status is. I am concerned that it may have been closed by Ebay if it looks as if you didn't send the item back in time.
Can you manually add the tracking to the case?
If not, and it has been delivered, contact Ebay Customer Services.
Use the link to get a callback.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)