Thus the all important need for knowledge. Why is knowledge so important, and what type of knowledge do you need? Any and all is useful, it all comes down to analysis, and what you can do with that knowledge. I'll list a few different types of information I've picked up from various, shall we say "sources" *conspiratorial laugh*.
One of the most basic needs is to know your server. Know when the various types of players are on. Know what different players do. When do guilds raid? How large is their guild? How good are they at raiding? How far are they currently? Are they working on bosses, or simply on farm? For this first example we will talk about my demographic, the raider.
This information is important. Since raid lockouts reset on Tuesday, many guilds raid on this day. More casual players do Baradin Hold, and don't forget that you can gain more conquest points on Tuesday too! Valor point cap for the previous week is reset, and people are doing Tol Barad for the weekly quest, and trying to keep the area so they can raid that quick fight.
Knowing all this, what can you deduct? One of the more obvious conclusions is that GEAR will be entering the game. Good! What do players do when they get a new shinny? Hopefully most will enchant it, and gem it up. Belt buckles, leatherworking leg enchants, gems, enchants, and more will be in demand. The raw materials will also be wanted, as more savvy shoppers will buy the mats and have a friend craft the needed item. Guilds may be purchasing Lavascale Catfish, Highland Guppies, and Fathom Eels for Fish Feasts. If they are progressing, many wipes will probably be forthcoming. (A few months ago, it took us 150 tries to down Heroic Maloriak...go ahead and laugh.) Some guilds will be OMGPANICING when they check and none are left in the guild bank, and will try to buy them up. Flasks will be purchases, although some guilds require people to bring their own, or provide them in the form of cauldrons. The same thing for potions. Last night, on the 3rd pull of Baleroc we were teaching 5-6 new people on 25man the fight. A mess up on the switch killed a few people. (I'm looking at your Mr. Death Knight who wasn't standing close enough to me when I was tanking the crystal... *hug*) and we ended up wiping with under 1 million HP, around .7%. This underscores the importance of a little extra damage. Many progression guilds pre pot, thus using 2 potions per pull. Twenty or so pulls a night, dozens of people pre potting, healers using potions of concentration...the numbers are staggering. Knowing whether guild buy these potions and flasks, or provide their own but buy the materials, or even farm the herbs themselves is important!
Thus, one of the first steps is knowledge is know your server. The two best ways I've found to doing this are to check The Undermine Journal, to see how often certain items sell, at what price, and what the range these items normally go for inside. With practice, you'll get to know when to sell hard, as the price is way inflated, and when to hold back. Example, belt buckles. On my server the materials cost me around 90 gold to make, on average. They sell for between 190-375 gold, with an average price of probably around 270g. Knowing when to post to get most sales, and to be the lowest priced seller is key, as is knowing how many sell, on average.
Knowledge is power. Know when guilds raid, and what they require so you can fill their needs. Understand demand, understand your server. Riches await. Know when many people log on, when demand is high, and what sells is a major key in being more then a greedy little goblin, it will lead you to the higher calling of being