Each trader generally sells a specific type of item, the higher the trader the more goods they have access to sell. This does not mean they cannot sell anything, but more of what they can buy themselves.
Here are the current trade groups:
Drinks - Commonly used by Tavern owners.
Food - Commonly used by Tavern owners.
Clothing (2 Stages) - Sold by clothing shops. Handled By Professions
Armor - (2 Stages) - Sold by armorer's. Handled By Professions
Weapons - (3 Stages) - Solder by arms dealers. Handled By Professions
General (Assorted stuff)- Sold by general store runners and merchants.
Note that this is for the ability to buy things from the trade menu, it does not stop you from selling other things\buying things from people and selling them on. Nor does it stop you producing your own items through labour.
Didn't I steam you something along the lines of tiers yesterday.