In either situation the tracking should show that the package was turned over to the shipping partner (USPS). That is what I have seen on recent purchases. Oh there have been a few cases where UPS or FedEx went ahead and delivered the item rather than turn it over to USPS.
If the tracking shows delivered and if the item was not received, then the buyer needs to go to USPS and find out the GPS coordinates where the package was delivered. GPS will have corresponding address. IF the address is yours then the item was picked up by another person in household or a victim of porch pirates - neither the responsibility of the seller.
If the item shows delivered and the GPS shows an address that was not yours, then the seller is responsible and you need to get a letter from USPS on letterhead stating where the item was delivered and signed by an employee that can be contacted, with phone number. Use that info with an INR case and eBay will help you get a refund from the seller. So tracking that shows delivered to the USPS as "delivered" to the buyer is a problem for the seller and a defensible claim for the buyer.
Now, personally I check all potential purchases and if the shipping is SurePost or SmartPost I avoid buying from that seller. One shipper is a mess two, one after the other..... Also, I never sell anything using those services and they appear unreliable, confusing and error prone and I do not wish to inflict that on my buyers.
