That brings up something that I wish to add to the whole store revival project as a venue for smaller/independent traders.
Example: I have a fortress of 150 dwarfs. 20 of these dwarfs would really really like some obsidian jewelry. Me, being the horrible overlord I am don't have any production orders for rock crafts - obsidian, and don't plan on making any since I'm a horrible overlord and don't notice this. Now, unhappy with the lack of obsidian crowns and the inability to jam 30 obsidian earrings in each ear, they swing by the market, 'Heyo Jimbobrandomtravelling McMerchant, do you have any obsidian crowns or earings?'.
Why, no, sorry, I don't. I have some of these other things though. Ah, but what happens is the travelling merchant makes a note, 'I had a demand for obsidian jewelry at outpost Flopping Deepshaft. So, the merchant wanders off to the world map, wanders around, visits world sites, does some trading with an eye at hopefully procuring some of the things on the wants list that has built up during the stay at your fort, and, fates willing, survives the hordes of goblins to make it back eventually to your fort with a general selection of items and (hopefully for the merchant) a more specific selection of things that where asked about.
This whole idea posits the ability to coherently manage world tasks with an entity, in this case a merchant (solo or caravan). Eh, something for the future.
And coins don't have to be metal. Let alone precious metal. Stone. Wood. Shells have been in vogue at times too. 'Currency' is a rather wobbly group consensus, eh