Placing your retainer in a certain ward reduces the tax on items within that catergory. For example, selling clothing with an optimal rank of 21-40 is best done in the Middle Tailor's Ward.

Normally, you lose 10% of your sale price to the tax/gil sink.
But, sell it in any market ward and it becomes a 5% tax.
And sell it in the correct ward and it becomes (I think) a 2% tax.

The reality though is that people just sell it where they can, since actually getting a retainer summoned in the wards is hard enough as it is with them almost always being 100% full.