There is a market that is an amazing and dependable investment. You can keep sinking your gold into it for amazing returns. That is Level 77-80 Cataclysm Greens. The reason for this is the fact that when players hit level 80 and want to queue for the Cataclysm dungeons the item level required to queue for them jumps up to 226. This major jump requires putting Cata Gear in the majority of gear slots(especially if the player is using the BoA helm/cloak) and people generally bite the bullet and pay it. Two other major reasons people buy this gear is to twink out toons for PvP or to twink them out for doing Wrath dungeons for the last couple levels. Personally I recently dropped a ton of coin on my Warrior tank alt when he hit 77 and was having a blast destroying the dps meters as a tank in Wrath dungeons. This gear sells over and over.
I use an Auctionator shopping list to search for these, and this is what my list looks like:
I used to only have the 4 Armor types plus Weapons but I realized those searches were missing the Off-Hand Crystals and Shields as well so I added those criteria in.
Some things to note:
*Try not to buy pieces that have to compete with crafted items that go in that slot. You're going to see quite a bit of Tsunami, Darkbrand, Hardened Obsidium etc pieces being put up and oftentimes those get so cheap it's hard to turn a profit on a piece of gear in that same slot, especially if the secondary stats match.
*Weapons are very tricky. Some weapon types are very hard to turn a profit on and others are amazing. Stay away from Scepters, Wands, and Spellblades for the most part as well as in general most 1-handed weapons. The best weapons to resell are Bows, Xbows, Guns, 2-Handed Weapons with Str on them, and Staffs with Int. Staffs with Agi CAN sell but usually have to compete against [Berto's Staff] which often gets very cheap on the AH due to high supply. Daggers with Agi can also sell decently but once again have to compete with [Toxidunk Dagger] which frequently hits rock bottom prices in my experience.
Going back to Bows/Xbows/Guns these hardly ever show up on the AH and are in high demand. It is very easy to buy one for 120g and resell it for 450+ because hunters will even buy a ranged weap with Str on it if it gets them the item level + wpn damage.
*Cloaks shows up on the Cloth search so I don't have a separate search for them. Cloaks can sell pretty easily as not very many of them show up on AH.
*Avoid "of the Whale" or "of the Beast" suffixes on gear. They suck. "Of the Tiger"(Agi/Hit) is usually also bad as it's mostly on Plate gear, though there are some Weapons and Cloaks you will see from time to time that have this suffix as well which isn't so bad. "Of the Invoker" lacks Stamina but has a second offensive stat(crit) replacing it which some people like so it's a mixed bag but I still find them to sell decently.
*Tank Shields have to compete with [Hardened Obsidium Shield] most of the time but I find that if a shield has Mastery on it you can still sell it for a lot to the smart tanks who want good stats. Caster Shields are easier to sell than tank Shields if and when cheap Hardened Obsidium Shields happen to be on AH.
This is how I buy/price my greens:
Cloth Armor - Buy @ 75g or less - Sell 195g
Leather - Buy @ 75g or less - Sell 195g
Mail - Buy 60g or less - Sell 195g (I find Mail gear to be more plentiful and harder to sell so I buy it for less)
Plate - Buy 100g or less - Sell 195g(if it has int on it) to 250g(str)
Cloaks - Buy 125g or less - Sell 250g
Crystal Off-Hands - Buy 125g or less - Sell 250g
Shields - Buy 175g or less - Sell 350g
2-Hand Str Weapons - Buy 200g or less - Sell 350g (very easy to resell)
Ranged Weap with Str - Buy 225g or less - Sell 450g
Agi Ranged Weap - Buy 400g or less - Sell 650g (extremely easy to sell even at high price)
Hope this was helpful! I've learned a lot from my forays into this market so if you have any questions feel free to ask them here or send me a private message. Let me know if there is something I haven't covered here or particular greens you've had great success with. May the goldz be with you!