The website is http://elitistjerks.com/forums.php and if you are at all serious about the PvE aspect of World of Warcraft, you should already have it bookmarked. Why? Because this is where all the the mathematical modeling magic takes place. Why are you stacking Strength as a Retribution paladin? Why do you hear about Enhancement shaman running around with spellpower weapons?* Why is that Unholy Death Knight wielding two 1H weapons and dealing 2,000 DPS more than you?* Because Elitest Jerks (hereafter EJ) said so. Fire mage rotations, Resto druid stat weights, and more can be had for free from people who enjoy simulators and spreadsheet more than you. The format is not always conducive to to finding summaries of the information you care about, but when they do summarize, they do it better than anyone else.
Whether or not you have the site bookmarked, you have probably heard the EJ haters. Maybe you even indulged in it a bit yourself. "If you aren't getting the socket bonuses, you're doing it wrong." "No thanks, I'm going to spec the way I want to." Sure, no problem. I mean, unless you care about maximizing your character's potential, there is no wrong way to spec. If your intention in picking one talent or enchant or gem over another was to improve performance, then there are, in fact, ways of doing it wrong.
Why bring the EJ forums up at all? Simple: the site is a really, really good market indicator.
I alluded to the Defender's Demonseye in my Transitive Property Farming post, for example. The Wrath version of that gem was complete garbage because Parry as a stat suffered diminishing returns faster than Dodge, so you were almost always better off using anything else in red sockets. That was Wrath. In Cataclysm, Parry and Dodge suffer diminishing returns at the same rate so balancing the two stats (11%/11% is better than 12%/10%) becomes very important. I did not know about the Defender's Demonseye before I was looking for something to put into a red socket for Protection paladins, and it was the EJ forums which indicated that the gem was useful at all. So, I bought the pattern, socketed my gear, and decided to toss a few extra on the AH for 150g since Purified Demonseyes were being tanked down to 60g. There was no competition for Defender's Demonseye. I listed some more for 175g. They all sold, usually to the same tank. Raised to 250g each. Still sold. It has been more than 30 days since Cataclysm has shipped and the hardcore JCs will have had gotten 10 cuts by now... and yet I have the Defender's Demonseye market all to myself. The tanks have done their EJ research, but apparently the (other) JCs have not.
Sometimes the EJ advice is not particularly useful to Auctioneers. "Stack Strength gems." Yeah, okay, there are 40 Bold Inferno Rubies going for less than the uncut gem at the moment. But if you dig a little deeper, you may surprise yourself. In browsing weapon enchant scrolls on the AH, I noticed Berserking still up there at 600g. That seems a bit silly, I thought. I tossed a few up and they sold. It was not until I looked at the stat weights for Retribution paladins that I realized that Haste was worth 0.27 per point whereas AP was worth 0.69. In other words, Berserking provides more DPS than Hurricane for Ret paladins by about 150 DPS (based simply on 400 AP vs 450 haste, not uptime considerations). The upcoming patch 4.0.6 will probably shake things up, but until it does, I will continue buying Abyss Crystals at 15g apiece and pumping out Berserking scrolls at 400g margins to the people doing their homework.
Do you have to understand why something sells before you sell it? Nope. But understanding the why of things helps you get an idea of how deep a market might be and when you may want to hop off the train before it hops the rails.
*Not after 4.0.6, of course.