Vendors and Tavernkeeps buy and sell at rates based on completion of a specific Reputation Quest. Talk to the City Ward greeter or contact for specific information on Reputation quest (or see full list below). Brokers have varying rates based on minimum level of items in which they deal (eg., ml:2/0 Broker refers to min level 2 weapons/armor brokers and min level 0 clothing/jewelry brokers).
Here is how Haggle impacts the buying and selling price of items, at the different vendors.

Example: A player character with 0 Haggle skill wishes to buy a ml:2 weapon from the Broker. The base value of the weapon is 8,000gp. The ml:2 Broker markup is +25%. So the player can BUY for 125% ItemBaseValue, or 10,000gp. The same player uses another character with 40 Haggle and can BUY the item for 100% base value, or 8,000gp.

Now you know better than to sell your hard earned loot to barkeeps and other similar vendors. Brokers are the way to go. They are the people that pay higher for your junks, err, items. Most importantly, DONT LET Mississippee catch you selling your hard earned "lootz" to the bartenders... Anyone caught doing this will be subject to Mississippee Queen's "MP3 BARD LAW". (15 minutes of listening to the loudest, most obnoxious MP3 files on my hard drive!!! ... at FULL VOLUME!)
SELLING = BASE SELL PERCENTAGE + (0.25% x HAGGLE)
BUYING = [1 - (0.005 x HAGGLE)] x Vendor or Broker Level MARKUP