Because you could have a SIGN. Even with the Trade Broker, crafters still use the Private Store to quickly unload mats in the main cities and have signs indicating the kinds of mats they specialize in. No one bothers going from private store to private store - if you see a sign with some items you want, you go there! If not, you pass by. It's realistic, as no one goes to a store with no idea of what they sell inside! If they're trying to emulate a real "bazaar" feeling in parts of the world where these are common, then you have people shouting out their goods, showing them off. You don't wander up to a random dude in a bazaar and say HI DO YOU HAVE ANY COTTON BALLS?
Yeah, the Private Store system is in no way as heavily used as the Trade Broker, but when the game launched in the US, when people were still in starter zones (with no AH access) there was heavy use of the Private Stores, and it was an awesome way to sell decent drops and find gear that was way better than what the NPCs were selling.
Also, because you didn't spend half your time crafting these items, it didn't really matter what they sold for. The Private Store economy was variable but it was easy to plunge into because, at low levels, it was mostly just for extra $$$ and fun.
tl;dr - signs would help, if you think the bazaar system could be salvaged.