The true secret to making money on WoW is learning to use the auction house. It takes a lot of business sense to really make it big on the auction house. Thankfully for many of us, there is an addon that allows even those who know nothing about the economy to do well on the auction house. I highly recommend this addon for everyone who ever uses the auction house. If you don't use the auction house, you're missing out on a lot of money.
Auctioneer is a very complete set of tools for dealing with auctions. You really only notice it when you use the auction house, but it adds multiple tabs and buttons to your auction page. First of all it scans the auction house and keeps track of the prices on items. It will give you the average prices that an item sells for and compare those to the items you are buying or selling.
When you want to buy an item, just search it normally. Auctioneer will list them in order based on how much of a bargain they are, and put a % next to the price showing how much it is today compared to the average price. When it's blue at say 30%, it's a great deal and you could probably re sell it another day for a profit. If it's red at say 150%, it's a total ripoff and you can save a lot of money if you wait until the prices drop.
When you want to sell an item and don't want to think too much about it, you can just drop it into the auction house and it will auto suggest a price for it based on the competition and prices for the day. It will put the price barely below the lowest bidder so you're most likely to sell the item.
You can also save custom searches and set it to remember prices so you can put up multiple items of the same kind without having to re-enter the prices. This alone saves me lots of time.
You can Download Auctioneer Here.