Enchanting is a profession with extremely high profit potential. There are many different markets you can sell to, and there's many different ways to make money within those markets. Of all the different ways to make gold with enchanting, one of the most lucrative is #3 on this list – profiting from the sale of Large Brilliant Shards, Illusion Dust, and Lesser/Greater Eternal Essence. These enchanting mats can be extremely pricy on the auction house as they are used for leveling enchanting to 300, and for twink enchants with no minimum level requirement. Here are a few great ways to get your hands on a bunch of these mats, and sell them to hungry AH buyers!
Tailoring, Leatherworking, Blacksmithing – if you can craft items to disenchant into the appropriate item levels cheaply enough, crafting can be a handy way to produce Illusion Dust and Eternal Essence. Using an add-on such as Auctioneer, add up the crafting cost for an item and compare it to the disenchant value. The best trick is finding cheap leather, ore/bars and cloth in order to make a decent profit!
Quests & Outdoor Farming
Doing quests in high level vanilla zones such as Silithus and the Blasted Lands will give you a healthy supply of green items to disenchant. Slay as many monsters as you can find, as they have a chance of dropping green items, cloth, and other loot.
Soloing Upper or Lower Blackrock Spire can net you upwards of 1,000g per run! Several large brilliant shards, greater eternal essences & illusion dust can be farmed off a single run of UBRS/LBRS. Especially if you are level 80 or higher, this is an incredibly easy way to make guaranteed, risk-free gold in under an hour. (Bonus points if your second profession is Skinning!)
The item level of the armor or weapon will determine what mats you get from disenchanting the item. The TradeSkillMaster add-on makes purchasing green items to disenchant incredibly easy. TSM automatically calculates the expected value of an item to be disenchanted, so you can compare unit costs. Blues of item level 55-65 will yield large brilliant shards, and greens of item level 56-65 will yield essence or dust. Armor will yield dust more often, and weapons will yield essence more often. If you can obtain armor/weapons cheaply enough, you can make a profit disenchanting them and reselling the mats.