The ' stupid' seller if they have a tracking number that says it has been delivered , is what they are being told by Royal Mail or by the courier used.
All you can do is to open a case for item not received from ther Resolution Centre.
https://resolutioncentre.ebay.co.uk
Escalate on day 4 if not refunded beforehand.
Be aware , if the seller enters tracking that shows delivery, they will not have to refund you. You then need to chase up where this item was left. It may be in your local sorting office, waiting for you to collect it.
