There are many markets in Ur-zitz, but predominantly of the following types:
In the Wall, there are markets to sell day to day items for use in households.
In the city proper, there are specialist markets that tend to trade exclusively in one type of good that have been made to be sold in shops - for example meat markets, furniture markets etc
However, the correct place to buy salt is the enormous floating market.
The city, if you only look at the land was once a crescent shape, around a circular natural bay. However, that bay now is entirely filled with ships that have voyaged out from the Old World or Veil, meeting here in the middle. The waterways to allow the boats through exist alongside great floating roadways to allow wagons to unload the boats, and there are berths along these roadways for the ships to dock at.
The entire bay is always in motion, roads are reconfigured to account for incoming vessels, and waterways are opened or closed to schedule, and the whole place is a hive of activity. It is the root of the wealth of the city and each ship is a market all to itself. Most will have sold all their wares by the time they reach their berth, which can take days, and have already organised and paid for the restocking for their return journey as deals are done as boats pass each other on the painfully slow journey through the waterways.
The main problem you will have is knowing which vessel has salt to sell, and at what price (If you care about the price at all!) but you can always hire a guide. Guides are used by nearly everyone who uses the market. They are inexpensive - not that this matters as none of you have any money - and can save you days of searching and a small fortune thanks to their knowledge of the incoming vessels and the best prices for what you want.