It has nothing to do with the seller or ebay
As you say you need to contact Royal Mail and sort it out with them.
It is difficult because they will often only deal with the person who sent the item your seller
If the item shows as Delivered ( even though it was left on the doorstep) then you would lose any dispute on ebay or paypal if you paid that way.
Are you sure it was left on the doorstep
Have you checked with neighbours.
In our area the RM do not usually leave on the step. Couriers do
