As I said before, I'm sieging Feldmere at the moment, but before I got to K-PX I already owned a market raid artifact as well as a market raid power (probably through the daily reward). So back then, I didn't know what a market was (and what market raids were for) and my imagination kicked in. What I'm about to propose now is one of the ideas I had of what a market might be.

What if there were objects on the map (just like there are mines, smelter, army camps, etc.) that you could capture/befriend/"unlock" that then could sell stuff for you at a high(er) price but similarly to the smelter/crafter that they would only sell so much units per time. I'll call this merchants for now, but the name is just to make talking about them easier in this post.
So around the map there are some merchants. If you capture the respective territory/bonus, you get access to one. If you have some, you can then offer them to buy something from you. Similarly to what markets actually are, they may buy some things for some amount of money. For example, one might be interested in copper stuff (copper bars, copper wires, whatever else there will be that makes use of copper one way or another) with higher prices than if you sell stuff from the respective tab.
But the merchant will only buy from you:
- if you have the amount he wants (each time he buys from you, he either takes say 12 copper bars or none if you haven't enough at that point, like a smelter he will wait until you have enough and will then take it)
- one specific thing you selected (just like a smelter will usually stick to one recipe at a time), but after each time he bought something, you are free to select another of his desired things (next time he should then buy wires instead of bars from you)
- after he is done selling the stuff he bought from you and this will take some amount of time (that will also be determined by the item he buys from you, the "cooldown" for selling copper bars to this merchant might be different (probably lower) than for copper wires)
- if you choose to sell at all (like with the smelter you can always decide to not sell at all ("deactivate"), e.g. when you intent to unlock a tech)

I'm still early in the whole idle game, but I feel that there is some point where every alloy/item eventually gets somewhat obsolete because you have enough of it, you need smelter capacity for other stuff, or it's just not worth it anymore to sell e.g. copper bars. Just like crafter can force you to set up your smelter in a specific way to supply them with ingredients, I think merchants could be an additional aspect to make you re-plan your gameplay. Don't having enough items for a merchant isn't inherently bad, but if you rely on additional money income, you might have to keep one smelter on copper bars to satiate the merchants, but then this smelter can't work on alloys you need for techs. Merchants might even offer different deals for the same item that will then be interesting for later stages in the game to switch to another deal (5 copper bars, 5k money, 5 mins - 50 bars, 75k money, 60 mins - 500 bars, 600k money, 400 mins - or whatever, I just wanted to emphasize here, that the same merchant might take the same good with varying prices per item and "cooldown" time per item, the last deal gives you the most money per time, but you are required to have more items per time to fulfill his needs). And merchants should always keep in mind what markets want for their stuff, buying from markets and selling through merchants shouldn't make you (too much) profit (some is fine because you have to supervise the situation to buy new items when the merchant has almost emptied your stock, and merchants don't instantly sell all your stock at once, so if you can make a slight profit, buying from markets and selling through merchants is in a way a form of investment in the future).