Market/Fairground?

[Quillan]

It's worked that way for me, but I haven't looked closely. I think it was the first market I built that let me build another merchant. Building more than one market may not allow for it. I might have to build a fairground before I can get another merchant.

[Jimmytwohand]

The better the markets, the better your merchants starting stats will be also allowing you to make more money proactively rather than just the passive trade bonus and extra income they give. They also seem to be vital in getting the higher levels of merchant guild which provides its own benefits.

[econ21]

Just to say, finally (turn 61), I had a case where the settlement details showed building a market would increase income. So the income effect can show up, but might require some advanced conditions. (For example, in BI, I believe building ports in some settlements did not increase trade income because their "twin" trading partner was held by an enemy - maybe something subtle going on like that.)

[majesticchapel]

I think you might only be allowed a set number of merchants over the entire range of your cities, in other words you can build 3 merchants in London, or one in Edinburgh, York and London each.
This number then increases once you build more merchant buildings.