Today let's look at disenchanting. Sure, we all knew that the huge spike in enchanting mats wouldn't last when the expansion just came out; I remember selling dusts for upwards of 800g per stack. Now, they're roughly 350g where I am. But that doesn't mean you shouldn't disenchant items.
First, you have to look at the cost of selling the item on the auction house, and disenchanting it. If you have a level 84 green item that gives crit, agility and hit rating, chances are some hunter or rogue will want it. If you have a level 84 green item that has only spirit, chances are it won't be in as big a demand. Now that we're done discussing common sense, you have to ask yourself, do you want to wait for 100g profit, or would you rather 50g profit now?
Greens may come back to your mailbox a few times before they're sold, and with them take a pretty substantial deposit (upwards of 1g per 12 hour post). You may sell that green for 100g, but after listing it 3 times that's 97g. It's important to not discount the cost of the deposit when listing items, although it may seem trivial. Usually, the deposit is small enough to not be noticed, but if you're listing an item 10+ times, it starts to cut into your profit. Personally, I just disenchant all my green items because it's easier for me to group them together and sell. If you're into micromanaging, you could list them all individually for a slightly higher profit.
So, how do we get greens? Well, let's look at the 3 options:
1.) Finding them
2.) Buying them
3.) Making them
1.) Finding them - You could grind, or run instances. I find maybe 1-2 per day doing my daily quests, but this isn't going to get you rich. Higher level enemies drop higher level greens.
2.) If you purchase a level 82-84 green off the Auction House, you'll more often than not get at least 2 Hypnotic Dust, if not 3. Thus, the rule of thumb here is to purchase any green item, level 82-84, that can be bought for twice the price of one dust.
Ex. 2 Hypnotic Dusts on my realm go for 30g. I buy any green item, level 82-84, for 29g. I disenchant them, and most of the time I'll make my gold back, but when I get 3 dust it's like 15g in my pocket.
3.) Making them. Every profession has a cheap green item to make, although it may not always be worth it. Again, be sure that the item you're making isn't worth more in raw materials (unless you need a skill-up), or worth more as the item itself than the dusts. Jewelcrafting is great for this: recently, prices of Jasper have dropped to roughly 9g. Add in the cost of 1 Jeweler's Setting, and you have 10.20g (at exalted with whatever vendor faction). Disenchanting this yields 1-2 Hypnotic Dust. So, if I get 1 dust, that's around 4g profit. If I get 2, that's roughly 20g profit. Solution here is to buy all the Jasper on the Auction House, disenchant it to dust that moves very quickly, pick up the gold from the inbox, and /dance. I still sell Hessonite and other gems because they are worth more as the gems than they are as the dusts.
-Disenchanting can be a very valuable way to make gold
-Buy level 82-84 greens selling for twice the price of Hypnotic Dust
-Make any greens that would be worth more as dusts than they would as items or raw materials