they could make this maket zone inter connected with all cities. And with in the market area. If absoluted needed have an Auction house area.

simple put you enter the market market zone via teleport. (maybe a similar design of a guildleve card but you see a stall. You enter a pretty shoping area and from here can enter any section from this area. each housing an outdoor area with retainer stalls. each enterance will have a counter where you can do an item search. you search for you item with a prince range, then just click one of the listen which teleports you to that floor its located on. (meaning each ward will have a few floors to limit over fulling. and if a floor gets full it will not be on the list) All you need to do is walk to the stall with the star over it.

the wards could either be organised by diciple, or just numbered. (since ppl will sell misc stuff) the opening section can be for personal bazaar to limit over crowding in the main cities. Broker npc can be placed in the back area.