A barter system sounds neat and makes more sense to me than some Duke guy who keeps minting new Dukes for me to smelt into brass and cast into bullet casings. Now that I think about it, shouldn't I be able to cast brass into Dukes? 
But I worry bartering would impractical to implement unless perhaps it was done in addition to Dukes and as an alternative form of trader quest. That is, the trader keeps a small list of X for Y entries. That sounds vaguely like Minecraft Villagers. 1 Gold Nugget for 5 Fish? Anyone? Anyone?