If you are serious about making Gold in World of Warcraft then you are definitely using the Auction House often. If you have a profession such as skinning or mining or you are farming for stacks of trade goods then you will certainly have quite a few stacks when you are ready to sell. When you get to the Auction House and see other people selling the same thing that you are the natural reaction is to price your items a bit lower than the lowest one, put them all up and then get back to what you were doing.
This is the last thing that you want to do! If you start listing tons of items below the lowest seller then you are effectively killing your own market. Once other sellers see a ton of stacks that you listed they will be forced to undercut you and continue the cycle. Even if you sell all of your goods you will not get the best prices for them and you may ruin the market for those items for quite a long time. Instead you should list for slightly below average prices and only list a couple at a time. If you are selling goods that are used for crafting professions they will sell out quickly anyways and you will make even more gold for the same time spend gathering or farming. And most importantly you will not ruin your chance to sell the same item in the future.
So in summation make sure that you are patient and you do not undercut others and flood the market. The long term costs far outweigh the short term gains.