It is far too late to do anything through Ebay and the seller is not prepared to help you. Royal Mail have told you that you are ineligible to claim and the seller won't make a claim. It doesn't really matter - even if 50 people told you that the seller should claim, you cannot force them to do so. They may already be too late to claim from RM in any case.
See if you can make a claim through your payment provider.
If tracking shows the item was returned, uncollected from the Click and Drop address, you may be eligible to open a charge back.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)