Forgot about how the main story pushes everyone to Ul'dah at one point.
Even though merging all the markets into one single market does seems odd, (like the FFXI chocobo races linking all the cities with a convenient portal) the application is one way of solving the issue.
The one lingering problem is how there is no time limits nor penalty by leaving a retainer on the market wards. What this allows is a pile of retainers that can be selling nothing. This happens just because the players wants to ensure a placeholder.
Just recently I went to look again on Selbina Ul'dah markets. Granted this random sample set is extremely small, but allows you to see at a glance.
I start at one end of the ward, take a few steps, and check a new floor retainer that first appears up on the screen, repeat until I reach the other end of the wards with a total of 10 retainers checked.
The following are the number of retainers selling nothing, or selling items only priced at 999,999,999;Tinkers 5 of 10
Battlecraft 1 of 10
Spellcraft 3 of 10
Fieldcraft 3 of 10
Tradescraft 1 of 10
Armorfitters 3 of 10
Lower Tailors 3 of 10
Middle Tailors 2 of 10
Upper Tailors 2 of 10
Jewelers 3 of 10
Grocers 3 of 10
Chirurgeons 0 of 10
Ironmongers 2 of 10
Woodcutters 0 of 10
Tanners 1 of 10
Masons 1 of 10
Clothiers 2 of 10
Crystaliers 1 of 10
Menders 1 of 10
Total 37 of 100 retainers.
That sounds like a lot, but remember it's a small data set. A better way to get more reliable numbers is to use yg's updater program and sweep all the retainers. Assuming, yg fixes that functionality eventually.
But even disregarding that, the issue there is people feel the need to have a placeholder. Why? Because marketing space is limited. Even if they don't plan on selling anything, the value of having the spot allows them to sell things easily. That'll still remain an issue even if they merge all the markets together while keeping the same market structure.