The broker listing is simply an easier way of finding what people are selling in their apartments.
When you list something in your apartment, it will automatically appear on the broker's list, and people can buy the item in one of three ways:
1) They can come to your apartment and buy the item from you at the list price. When you list something, your apartment will default to accept visitors automatically, unless you already had your apartment open to visitors.
2) They can buy the item from a standard broker in their city. This broker only lists items sold in that city and will mark up items by 20% as a sort of "finder's fee" - the seller does not get the extra 20%.
3) Players from the other city can buy your item using the black market broker. This broker lists items from both cities and charges an extra 40% compared to the list price of the item. Again, the seller does not receive any of the money. This is the only way to buy items from someone in the other city, aside from directly buying from the other player in an adventure zone.