Every few years a new XPac arrives for us to take the opportunity to gouge the hell out of everyone. What does this one encompass?
Well first its regarding the guild achievements. Everyone is going to want a dark phoenix mount and the requirements to get it is getting all the legendaries in the game. You have to however get the items while cataclysm is launched meaning there is a huge market for this item. The item i am talking about?
Item
Arcane Crystals
While other guides mention this vaguely the reality is, this is probably going to net you a decent amount. While i am saying this it is all theory but what i have learned from experience is that most of these old world mats end up getting gouged nicely.
Why Arcane Crystals?
Pretty simple actually, this is one of the hardest mats to get. (Unless it's drop rate has been upped). It cannot be prospected! So you have to constantly be mining it from thorium ore. Now the last bit it is going to be needed for Sulfuron's Hammer which is a pre-req for Sulfuras, Hand of Ragnaros. (Sorry i don't have links wowhead down). They all use arcanite bars, thus needing arcane crystals. There is a low buy for them at like 10-15g, depending on how bad you buy everything on their. Sometimes best to leave one in the AH so when others post they undercut the price on the AH.
Summary
Buy Arcane Crystals at low cost 10-15g?
Sell for an outrageous price when cataclysm hits!