eBay's tracking is sometimes not in sync with Royal Mail // courier's tracking.
Was this a RM delivery? Check on their Track & race online to see where and when this item was delivered, or ask your postie where they left it, it could be in a ' safe place' or with a neighbour. Unfortunately, items if left in full view of passers by can get stolen.
Be aware, once notified the item has been returned, you have 48 hours to refund your buyer through the case they opened, or they can escalate and eBay will force the refund.
@mbuo_0