Using Mason's Ward as an example
Market ward style, but get rid of Retainers. Just have it so you can put stuff into storage from a Bank NPC. From there you can put it up for sale by selecting it's area (Mason's Ward for Ore) When someone ports to a certain market area, there is a single NPC you talk to (Mason Ward Officer) who shows you what is for sale. There should be different menu's to divide up the options, ore, gems, stone, so you can find exactly what you are looking for.
I really like both of my retainers, but sometimes I just feel like they aren't pulling their weight. So if they were replaced by a Bank Officer to look after my goods, I guess I'd be ok with that.
I can't wait to see what the Dev's do come up with though. It's just hard to imagine a Market with 20,000 NPC's all crammmed in one ward. Doesn't seem very fair to the Retainers. Like Sardines in a can