Even with Post Office insurance or Private insurance, the onus for proof still comes down o the seller.
And with post offices, stamps cannot be insured, whether by register mail or by small packet or an expedited type of shipment.
The only option for really expensive stamps to be insured is through private insurance of the seller's own.
Otherwise, it is back to the 25c a shipment placed into the pickle jar, to save up for any possible loss incurred.
International mail order (which
ebay selling is) has a certain element of risk and loss to it that, over time, the sellers recognize and take steps to avoid, as best they can.
The best insurance is to charge more for the stamps and shipping, unfortunately for the buyer of course, so as to cover projected losses, which do occur.
Somewhat like the old approval services. Buy cheap and sell dear, and hope for the best.
I shivered every time I had a sale of over $50 to mail out worldwide, or even within Canada. Not really for the postal services handling, but for the common human accidents that happen every day.
In Canada, registered mail only proves that a shipment (letter or package) left somewhere, and then arrived somewhere. That's it. But, it does make me feel better somehow.