I sort of disagree with the idea that coin markets would only be useful to top-end players. It also allows mid level players who produce bulk goods (linen, bricks, food, foragables) a way to save up for something good. As it stands if you want something valuable like high Q rings or metal for a brickwall, you'd have to wrangle a massive trade or figure out a credit deal and it's just a big headache for everyone involved. Ideally with a market you just produce a load of goods, trade it for coins, and repeat until you have enough to buy whatever.
Of course, if access to the market is strictly restricted that kinda puts a damper on that idea, unless people want to set up secondary markets or act as brokers or whatnot.
Regardless, I'm in favor of the idea in general because if it succeeds it'll be cool and if it fails it's pretty much bound to fail in a way that makes the game more interesting.