Barter is a self-enforcing protocol: nobody needs to facilitate the transaction or resolve disputes. It just works.
Except when bartering, participants do gain an advantage from cheating.
If I barter you my old laptop that crashes when you leave it on more than an hour, you're going to be mad later when you find out I didn't tell you, and you'll have to appeal to a third party for justice if I'm bigger or stronger than you.
EDIT: This is known as "deterrence" if the potential malefactor is fairly bright and "evolution in action" if not.
So barter should work for metals traders but not cattle traders (but they have other problems...).