If the seller has tracking to show delivery, eBay have to believe this, otherwise it's not worthwhile paying extra for the tracking service.
Could the item have been left in an unsafe place? Items do get stolen if left for viewing by passers by.
Have you asked neighbours, even household members if they maybe took delivery and didn't tell you?
Not as silly or impossible as that sounds, 2 of my buyers admitted when asking at home , that yes their items had been delivered handed to ' xyz ' at home , cases closed in my favour.
End of the day, if none of the above applies, tracking showing delivery is all the proof eBay need to close the case in favour of the seller.
@sidstevo