[Mithorium]

The evil seller doesn't ship the item but signs the product shipped message anyway. Who's gonna stop them, it's not like the blockchain knows if they actually shipped it or not. Or maybe they shipped a fake or non working item. Then what?

[decentralised]

The evil seller has a reputation to protect and the victim has an undeletable evidence of the "crime" on-chain for anyone to verify.

[Laforet]

How could you tell who is the culprit in this event?

[decentralised]

PKI.