As it seems that outside traders will be coming only once a year, in the future, in a yearly Trade Fair, I am suggesting Market Money.
In other words, it will give the visiting merchants, farmers, so on, a way to trade with the Fortress without rare coins or complex bartering. I am assuming that many of these traders will have items I don't want but many of my residents (or even other visiting caravans) may want.
In other words, I am assuming my Broker will be making deals under my orders while the rest of the Fortress also have access to the traders. In which case they can offer services or their own goods for trade. But what if the visiting trader does not want a vomit covered sock but he has a item one of the Cooks want. But said trader wants an item somebody else wants but that person does not want your sock. And that person wants something somebody else...so-on, so-forth.
It is kind of a form of IOU/Credit - let this video (which is not mine) explain.