Auctioneer is simply a must-have for anyone who spends time in the auction house. With this addon, players scan the auction house periodically and essentially carry a price list with them wherever they travel.

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Curious to see how much that green drop might sell for on the AH? Simply mouse over it with Auctioneer running and it will give you a recommended sale price, as well as an estimated amount of gold you could earn by simply disenchanting the item and selling the results.
Auctioneer is more than an automated price checker, though. It also automatically fills in recommended sale prices when you’re posting auctions (a huge time saver), helps you find bargain items on the AH that can be quickly resold for a profit, and gives detailed item information for just about every drop in the game.
The downside to Auctioneer is the amount of resources it consumes. I find that once you’ve got a good-sized database going, it can really slow down your experience on anything but a top of the line computer. I personally only use it on my bank/auction character and simply send him all the drops I wish to sell.
I highly recommend Auctioneer for anyone who likes to earn gold (and who doesn’t?)