I like the idea of building market mechanics. Maybe market stalls, like “carpenter’s stall” for buying and selling wooden goods, “stonemason’s stall” for stone goods, etc. etc. on to food, and other goods each with their own stall.
I think more than just building them, and bam they are usable, (boring and too easy) it would be cool if you had to build them AND meet town size requirements, or complete quests (!) for them to be usable. OR OR! Like you have to complete a quest for each merchant, and the first quest for the ironworks merchant only unlocks buying and selling a couple iron things, then there’s another quest that’s a higher level to unlock more stuff, and then a third or fourth quest to unlock more and more! 