Go to any NPC labeled "broker" There is one in every crafting instance and usually major city zones. In city zones, ask a guard for "Broker" he will tell you where to go with a glowy trail.
Double click the broker, To buy, open the buy tab and start searching. To Sell, open the "Sell" tab, place a container in one of the slots, then put the items you want to sell in the container. Then you can price them as you wish. You can use any container, a bag, strongbox or sales containers (Salesman's Crate, Weapon Rack, Armor Rack, Potion Container, etc.). If you own a home (even an Inn Room) you can place sales containers in your room, where players can visit you, buy directly from the container and avoid the 20% broker fee.
The "Good and Bad brokers" you mention probably refer to Qeynos and Freeport markets. If you're in Qeynos the regular brokers only show items Qeynosian players are selling. You have to go to the Freeport fence to buy items from freeport players, with a 40% markup, unless you can manage to sneak into their city and house. (I think that's possible, never tried it.)