This addon allows you to list items in your inventory at a specific price (or best offer) permanently (as long as they remain in your inventory).
A search form lets you query the current zone (or server) for other players who have the item you are looking for. Clicking on a search result connects you with the player owning the item, facilitating trade.
This allows you to buy/sell items in the field as is convenient, avoiding the auction house fee and the need to travel to an auction house hub. Particularly useful for quest items, health potions, or other items that would otherwise be more convenient to farm than make a special trip to buy from the auction house.