Most of the ways I make gold in WoW is through the simple concept of buying and selling the product through "flipping" it. This method is widely used, but very few understand the intracacies of what, when, where and how to do it to maximize profits. So here are some simple rules to follow or guide you in the process of flipping.
First of all, know your market. I used to be against the idea of buying and flipping armor for example, but when Blizzard added the ability to Transmorgify armor it changed the game. I only do small amounts of armor, but knowing which pieces are in demand and having patience to relist them a few times can pay off big time. I have purchased items for under 25 gold and simply relisted them for 500-1000 gold and they have sold. I could write up an entire method on just flipping armor and weapons because the market has changed so much.
Second, be willing to relist items many times. A good example is Copper Bars. I love to buy out Copper Ore and Copper Bars then relist them at higher rates. I have two strategies here, price them high enough that I make lots of gold but not so high that no one buys them; and price the Copper Ore higher than the Copper Bars so that people feel like they are getting a deal to just buy the Bars. Copper Bars will usually sell for about 1g per bar or less until I buy them and relist them for 2.5g per bar. This equates to about a 150% profit or more, but it does take patience because they are not a high demand item.
Third, be willing to buy more even after you have bought and flipped. Be careful, this is where most people go wrong. They come in and buy out the Copper Bars, relist them for a higher rate and then see that someone has come to the market and undercut them or filled the low end pricing of 1g each or less. So what do you do? Buy it. If you don't have the stomach for this then don't try flipping. There have been times I have bought Copper Bars for 4 or 5 days straight without selling a single stack. Most people give up and then relist lower and make very little or no profit. This cannot be done unless you are willing to buy, buy, buy. Eventually the market will stall, your copper will be bought and you will make tons of profits.
Fourth, be willing to cancel auctions and relist again. Ok, so now you bought the Copper Bars and relisted them for 2.5g each and some guy undercuts you buy a copper, what do you do? Well it depends on how many they listed. If they listed a stack or two then I would leave them alone and let them be happy. Chances are you will never see them again and they just listed based on what the current market showed. However, if they listed several stacks then you may want to go ahead and cancel your auctions and relist them by undercutting the competition. I usually do not bother undercutting unless I see it is a regular competitor of mine or someone posted a ton of stacks under mine.
Fifth, be willing to wait. Sales happen in spurs, do not relist items thinking they will all sell overnight because if you do you will be disappointed.
Sixth, you do not need to undercut all the time. If the item you are selling is in high demand then undercutting is only temporary, if enough of them sell then the buyers will eventually reach you so don't worry about it. This really wraps back up to knowing your market, if you know what the demand looks like then you will know what to expect, what to price them at and how long it will take.
I will elaborate further on this topic, but this will give you a start at how to flip items in the AH. Good luck and happy auctioneering!