Commentary for Kiddushin 56:1

Rav Judah explains the mishnah—for an item to be used as barter it must be something that does not have a value that can be expressed in its own terms. For instance, I could say a Hershey Bar is worth four Hershey kisses or two Goldenberg’s chews (I really loved those). But I cannot say how much a Hershey bar is worth in terms of Hershey bars. But a coin’s value is expressed on its own terms—a dollar is worth a dollar. Therefore, coins cannot be used in barter.