1. I take a gold hammer to the "Master Trader" NPC. I type in that I want it to sell for 1720 gold.
2. Now the "Melee Weapons" NPC has my gold hammer in his inventory.
3. Another player, shopping at the bazaar, sees my item, decides it's a good price, and purchases it. Now I'm 1720 gold richer (alternatively, the "Master Trader" gets a 5% cut or something).
4. If no one buys my hammer for 5 days, then the hammer is returned to me. Perhaps I should lower the price a little...
This system has a number of advantages. First of all, if you want to sell something, you dont have to stand in the crowd at Lion's Arch and yell it over and over again. For the buyer, you know exactly where to go to buy the item you need. Now you dont need to stand in Lion's Arch and listen to the loud people trying to sell their iron ingots and yellow dyes.
I really think this is a good idea. In fact I think it's so good I can't believe the programmers haven't already thought of it
