Earning money in World of Warcraft, the most widely played RPG in the world, is just another of the many challenges you can accept in this great adventure.
There are different monetary resources in WoW: doing quests, selling items to vendors or to other players and, of course, fiddling with auction houses. The point is, do you really know how to make profit of the game's auction system?
The answer is Auctioneer: this add-on will provide you with a lot of information about the items in your backpack: how many times have they been seen in auctions, what were the hightest and lowest bids, suggested selling price to make profit, etc.
All these data will definitely help you to decide if it's worth selling your items in the auction house or if it's a better option to drop by your nearest local vendor and empty your backpack there.