I noticed alot of people asking to improve the system, but I sensed this system is more likely need player dedicate their time in it, to do research on their own and try to find the best bargain, at least it was, now it's kinda stuck in the middle. What I think is you have to give people a better reason to actually spend time on it instead of leveling their job doing crafts and gathering. Players are asking for a simplified system because they don't care about the trading part but more focus on the production part for now. They only need a AH satisfy basic sell and buy function. For such a complicated trading system, it's too much for normal players to adapt, thus an introduction of merchant is required.

Merchant could be just your retainter, when you walk in market ward, pick one retainer and make him/her a merchant, then you can control the char while your main char stay in market ward till you finish

Merchants only have access to their own cities, they can only talk to npcs selling or buying stuff or check bazzar, but they have several special ability as

1, can go across servers freely. 18 servers are like 18 planets on the same orbit, in a circle. Let's say a retainer in server Besaid, might take 2 days to move to Wutai but 18 days to Lindblum. the price of the merchandise might change alot during the time he gets there

2, can affect npc price but not stock. you can push an item price to rocket high or dump an item price to history low, within the limit SE set up, but they will never be out of stock. Of course, other playes will use their retainers to find the best bargain across servers

3, if possible introduce a investment system, which can upgrade facility like guild or own a facility in other servers kinda

These concept are not new, I just think if you could make trading as much fun as other things, then players will dedicate time to actually follow your old plan, to actually find out the best bargain themselves, to be a winner in the market but not asking for a lame AH like system like old days.