This mod adds three new, unobtainable items to the game: Mithril bits, Bars of Mithril and Mithril Blocks, to act as a sort of "currency item", independant of inflation caused by player mob grinders [when using iron or gold for currency], xrays [when using diamonds], giant sugarcane farms [when using villagers] and so on.
The mod itself only adds the items - unless you want the trade to be purely player exchange based, you will need an additional shopping mod that lets you barter one item for another [like Simple Shops, or any kind of a chest shop]
The items [unless you script them otherwise] are unobtainable in survival, you need to be an admin in order to spawn them in, which means you are the one to control how much of the currency is in the world.