Picking the correct markets and then actually selling the associated items are two invaluable abilities on the auction house. However, if you don't master the art of picking out the right markets then the latter skill is useless and you'll be left spinning aimlessly in circles.
The world of cataclysm will be a rather diverse one and it will behoove you to read markco-vian economics before jumping into the art of picking your markets. Now, once you've become aclamated with my philosphies on long tail markets vs hits in cataclysm then it's time to narrow down your options into three possibilities:
- Niche Markets
- Frequent Sellers
Niche markets are perfectly wonderful on smaller servers when you're lucky enough to possess a rare pattern or just be the first person with one. These markets tend to eventually become crowded once more people hear about them (cough, cough) but can provide a very nice amount of gold while you control them. As more people enter a niche market competition tends to force the price lower and lower but depending on demand you could theoretically continue to make a decent profit, just not as much as you did before when you were lucky enough to be the lone seller. Niche markets are something you invest in moderately and should never have tons of stock waiting to be sold for the simple fact that tomorrow there could be fifty other sellers who heard about your technique. I would suggest getting into these if you can because they can be awesome money makers but don't over do it and don't carry excess stock around.
Frequent sellers are where the real money is made, which is surprising considering the fact that there is usually lots of competition, undercutting and smaller profit margins than the niche markets. Having thousands of small to medium auctions selling every night will make you rich rather quickly and allow you to reinvest the money into getting even more items to sell. The bulk of your money should come from these items and you should keep lots of stock handy for when there are spikes in demand. Knowing when these spikes occur and when to buy cheap are key to mastering frequent sellers in cataclysm.
Epics are a fickle lot. They take a good understanding of why people would buy the epic, how much they would spend and some simple math when it comes to estimating how much you will lose from deposit fees and the auction house cut of your sale. Due to these reasons I would only recommend that you get into epics when you have lots of gold, as in 100k+ and even then you need to be careful because epics will tie up your gold for a long time while they wait to be sold. Players tend to jump into these before really understanding the auction house and get burned by the deposit fees and final transaction fee. Epics should make up more and more of your sales once you have the gold and knowledge to maintain such a flipping enterprise.
What items of these three categories do you sell? Do you diversify or stick to one category?