Royal Mail have a streamlined process for EBAY sales - you need to
provide proof of the EBAY sale and the price the item was sold for. They
use that to determine the compensation level. All online and very easy
if the parcel is lost.

If it is damaged you have to provide photographic evidence of the damage
and the packaging

So you refund the buyer in full for loss or non delivery, and claim from
Royal Mail

I am selling of our lifetimes book collection, having retired years ago
and I can;t take them with me - so do not have purchase receipts - but
what you claim is the EBAY selling price