Auction Mods

AlreadyKnown villiv

Colorizes items that you already know in default merchant and auction frames.
WoWInterface

Auctionator Zirco

Figuring out a good buyout price when posting auctions can be tedious and time-consuming. If you're like most people, you first browse the current auctions to get a sense of how much your item is currently selling for. Then you undercut the lowest price by a bit. If you're creating multiple auctions you're bouncing back and forth between the Browse tab and the Auctions tab, doing lots of division in your head, and doing lots of clicking and typing.
Curse, Auctionator
Review: WoW Insider
Guide: Youtube Video

Auctioneer Nechckn, MentalPower, Norganna

The most popular auction mod. Provides you with in-game data on auction house items. Provides methods to identify bargains by identifying price trends and then finding items that will buyout or bidout for less than the trend data suggests should be it's correct price.
WoWInterface, Curse, Auctioneeraddon

AuctionLite Merial

AuctionLite is a simple, lightweight addon that tracks auction house data and improves the interface for buying and selling items. AuctionLite allows you to: Scan the auction house, Search for and buy out auctions, Create auctions based on the current competition, Display tooltips with auction and vendor data.
Curse, WoWInterface
Review: WoW Insider

AuctionMaster Udorn, Blackfyre42

A compact auction house tool for being able to sell items like the wind, scanning auctions for item statistics, searching for wanted items in the auction house, looking for good opportunities among the current auctions and displaying the statistics gathered so far in the tooltip of any item.
Curse

GnomishAuctionShrinker Tekkub

GnomishAuctionShrinker creates a simple, compact auction browse view. GAS is designed to provide an interface similar to Auctioneer’s “CompactUI” in a small standalone addon.
GitHub

MarketWatcher Torrid

Market Watcher is an addon that scans the auction house and records data for specified items and assists in daily auction house shopping in general. When the addon is set to record, a price history can be displayed to assist in determining a value for items of interest. What makes Market Watcher unique from other auction house addons is that it records multiple scans instead of just the most recent. This obviously uses a great deal of memory, so only scans of items specified by the user will be recorded. The saving of multiple scans allows for a more accurate determination of value, and allows the addon to deduce if some auctions were actually sold, so you can see which auctions are actually selling as opposed to basing a value off the prices of auctions that may not be selling at all.
Curse

TradeSkillMaster sapu94, cente, mischanix

TradeSkillMaster is a MODULE BASED system to help both NEW Auctioneers and the MOST ELITE AUCTION GOBLINS manage everything related to the Auction House! Our goal is to work hand-in-hand with our community to develop an all-inclusive suite of modules that, when used together, follows our users through every single auction and crafting related function, making them as much gold in as automated of an environment as possible while still following blizz Terms of Service very closely.
Curse
Review: WoW Insider

Undermine Journal GE Erorus

This addon is very simple. Install it, and most items that can be sold on the Auction House will have pricing statistics added to their tooltips. You do not need to run auction house scans like other pricing statistics addons, nor are there any options to set. Just install it, and you're done.
Undermine Journal

UnitPrice xbeeps

Shows Per Unit Pricing for Stacked Items on the Auction House.
Config: /up
Curse