In the previous post, we have established that player's needs drive the economy, turning them from Players to Buyers.
And the gold-maker's objective is to be able to effectively preempt them, stock up accordingly and post items at the right moment to maximize profits.
So what do players need ? Everything.
Okay, I know, that doesn't seem very helpful and this next part may be very obvious, but just follow me for a sec.
What is the purpose of all these items ?
What drives players to acquire these items ?
To answer this, you need to take a look at what players do in-game.
If you break down their activities, you'll be able to see categories of playstyles, routines, progress of characters and guilds that can define markets and what they want to spend their money on.
So here's a few markets that might be considered:
Leveling gear: Either items that have been picked up or bought or crafted with the intention of selling to people who are leveling their characters. Learn the different types of gear and stats people use to level. ("Green item of the Thief" will sell at a goodly price. "Green item with only Spirit" will give you a better profit if you sell to vendor or if you disenchant and use the result to make gold.)
Leveling trade skills: Items that are required to level trade skills, duh. However, there is a certain science to it - one has to know which are going to be the items in demand for each trade skill. You have to look at the progression of the profession (and Blizzard has a tendency to change that at each expansion) because players will ask for the quickest way to powerlevel the trade skill and the items will have to be on the AH right then and there. Hint: http://www.wow-professions.com/ People will heavily follow the guides posted there and it has all the info you need to know: number of items, tips on the hard-to-get ones, recipes to get, etc. It is a shopping list for easy gold.
People who need trade skill items: This is a big one and I will not go into detail about each profession. There are plenty of gold-making blogs that will go in very deeply into the analysis of player needs. I'll just say this: players need glyphs, and when they get to raiding they will need enchants on their gear, tailor or leatherworker armor kits, jewels in their sockets and flasks to bump themselves up. These are the sure markets you know people will have to buy from at some time in their road to max level or at end-game raiding.
Twinking: Twinks are characters that have stopped gaining XP and will try to get the best of everything they can get at their level. In 99% of all cases, they do PvP, otherwise they are a part of a guild that wants to experience WoW at a certain level, either because they don't want to buy the latest expansion or they want to play the game in a more nostalgic way. Either way, a gold-maker will take the opportunity to do the twink's homework and put on sale the Best-in-Slot items for them.
Mounts, Pets and Legendaries: Collector's items with achievements tied to them. Their are sites dedicated to those collections with step-by-step instructions on how to complete them. The items are most used for bragging rights, for hoarding or completionist players who want to have everything they can collect in-game or for Role-Playing purposes. One of the easiest ways to make gold, since a lot of pets can be bought from vendors.
World Events and Achievements: A gold-maker will already be collecting items for World Events months before they actually happen. There are specific achievements for the World Events that require certain items and there's plenty of gold to be made in the AH. Other more general Achievements are the same, but it simply comes down to taking the time to read up on those on Wowhead to take advantage of that information.
And there are more markets out there!
These are just a bunch of general markets that are tried-and-true and still keep making gold.
You just have to study those, find a specific part of a market and figure out how to get the max profit out of it.
For experienced gold-makers who already have a huge chunk of the knowledge wired in, it's all about finding new ways to discover or exploit markets, depending on what changes are about to come in the game or player's behaviors in the Auction House.
And trust me gold-makers are a creative bunch of folks.