Auctioneer NPCs can be found in in Orgrimmar, Undercity, and Thunder Bluff for Horde player-created auctions and are a part of the same Auction House. Alliance players will find Auction Houses in Ironforge, Stormwind City, and Darnassus. These Auction Houses are linked together for each faction. Horde and Alliance players cannot buy or sell items to each other unless they use the neutral Auction Houses listed below. Ask a guard for directions.

One non-faction based Auction Houses are located in Tanaris, Everlook, and Booty Bay These Auctioneer NPCs have the same functionality as the aforementioned faction based Auction Houses, with the additional capability of allowing trade between Horde and Alliance characters.

Sellers are able to place items up for auction, set a starting price, set the auction duration, and include a buyout price by speaking with an Auctioneer.

All auctions will be initially charged a deposit fee. The deposit fee is refunded upon the successful sale of the item. All successful sales are charged an auction consignment fee which is extracted directly from the final sale price. All fees are higher in Gadgetzan.

Buyers are able to search through items up for auction, place bids, and purchase items that include buyout prices by speaking with the Auctioneer. Players are not able to place bids on items being sold by a character on their own account.

All items won at an Auction House will be delivered through the in-game mail system. Auction buyers/sellers receive a detailed invoice of the transaction.

Browsing Auction Items
When you click on the Auctioneer you will see a blank list. You need to first search for what types of items you want to find. There are many possible custom search options including:

Name - Name of a specific item you're looking for.
Level Range - Set the minimum and maximum level of the item. For example, if you're level 30, try looking for items up to level 30.
Useable Items - Click this box if you only want to see items that you can currently use. That way you won't see Warrior items if you're a Mage.
Filters
There are many filters and sub filters you apply to your searches. For example, if you click on Weapon and press the Search button you will receive a list of Weapons for sale. You can dig deeper and select even more specific items using the filters. If you don't select any filter it will show you a random selection of items.
Weapon
Armor
Container - Bags
Consumable - Things like Potions or Armor Kits
Trade Goods - Things like Gems and Metals like Bronze
Projectile - Ammo for Guns or Bows
Quiver - Bags to store Ammo
Recipe - Trade Skill Recipes
Reagent - Things used for Trade Skill Recipes
Miscellaneous - Other things people sell.
Bidding on an Item (Buying)