Check the proof of posting you got from the Post Office when you sent the item and make sure you have added the tracking to the case.
If it was added when you sent the item and now shows delivery, you can ask Ebay to step in through the case.
Or:
Check the tracking yourself. If it shows delivered, you can contact Ebay Customer Services, make sure they have the tracking for the returned item tell them it was delivered 8 days ago and ask them to step in and refund you.
If the seller does not refund you voluntarily, you do have to return to the case and ask Ebay to step in. This won't happen automatically.
"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)