If tracking shows as delivered, it is ebay who rely on that , it is not up to the seller.
If Royal Mail you take it up with RM, it is their local delivery office and up to them to investigate. If a Courier then more of a problem, as they will normally only deal with seller who has no need to do anything.
You do not understand that Amazon is not like Ebay, Amazon sells and delivers, Ebay provide a means for any registered member to sell and arrange delivery themselves. Far too many use couriers who have terrible delivery records.