When I send a letter or a package and want to be sure it was received, I can rely on tracking or I can go the extra mile and request a signed return-receipt. Now all I have is confirmation the letter or package was handed over and someone confirmed they got it. They can still claim when they opened it, it was nothing but empty pages or a couple of stones - and there is no proof I actually put an iphone or important documents in there. There is nothing that guarantees both the seller AND the buyer that the actual item was very well in there and it was just as advertised in the auction and there is no easy way to prove it either.
From a legal point of view, how could you solve this? Let a third, independent and reliable party handle the transaction for you? Or do you pay a notary to certify that you actually packaged the iphone and that it's condition is as advertised?