How many retainers an individual ward can hold is determined by whatever load the server (computer tower at Square Enix headquarters) can tolerate without crashing and removing everyone's retainer. They have had quite a bit of testing done on this and appear to have gotten it to a stable maximum. Insofar as I am aware, each ward in each city should have the same retainer capacity that is independent from every other city and ward.

That being said, the Ul'dah wards are the most frequented and people rarely browse what is on sale in Gridania or Limsa Lominsa. This is because earlier in the game before many tools and features were implemented, players unilaterally selected Ul'dah as a central hub for the game's economy due to it's location, city layout and quest selections.

Putting up a retainer in Limsa or Gridania is not likely to achieve a sale.