I am going to write this quick little guide telling you the underfarmed matts that have made me lots of gold!
Purple Lotus
This is a very underfarmed herb that has made me rich in the last couple of weeks. To farm this item you must have 210+ herbalism, and be at least level 50 (recommended). The best spot to farm this herb is located in Jintha, in the Hinterlands
(It's Number 7 On This Map)
The thing that makes this spot so great to farm is that no one else is farming it! I manage to sell this herb for 20g Per 5 (80g per stack) when there is no competition on the auction house. I also buy this herb and resell it for more gold coinz.
Primal Air
Primal Air sells for mad crazy on my server! The thing is, you only see few air elements while leveling up in the outlands, and now with people leaving outlands early, primal airs aren't going into the auction house (Unlike Primal Shadow, Fire, and Life). I manage to sell these for 30gea, a decent amount. The best place that I found for farming these is in Shadowmoon Valley,
(The highlighted parts are the elements spawn points). I hop back and forth from top to bottom where they spawn, sense I kill them so fast. If you don't have a epic flyer, it may not be worth it though.
Another great spot is the elemental Plataea in Nagrand
This spot can be OK, you will have to kill some fires, and maybe some waters if your level 80 (going to be killing them really fast)
If you are a alchemist, you can get primal airs from the recipe Recipe: Transmute Primal Water to Air - Thottbot: World of Warcraft, it turns a primal water into primal air. Which is huge profit.
If you are a Engineer, you can also farm gas clouds in nagrand, which works out great. It's best to pick these up while mining nagrand, but if your just a engineer not a miner I suggest using
It works really well.