Proof of delivery, postal insurance ect is all for the seller's protection.
ebay already has systems in place to protect the buyer. Each seller has to determine for himself if he needs protection on lots shipped.
The determination is based on $ value of the shipment, prior experience with the buyer and a number of other factors only each of us can decide.
On shipments not worth the extra expense, we need to use the
ebay feedback system to help track that an item has been delivered. If the buyer doesn't give feedback in an adequate time frame, a follow up message should be sent to confirm that everything is OK. At least it will create a paper trail should a problem arise.
If that's too much bother for the seller, then the value of the lot isn't worth the effort and don't worry about it.
The vast majority of buyers and sellers are honest. It won't take long to identify those that aren't and to avoid them.