Here’s a list of what I keep in a bag on my AH alt for easy reference, so I can just shift+click the item, instead of trying to remember what to look up. Citrine, Jade, Aquamarine, Lesser Moonstone, Moss Agate, Star Ruby, Shadowgem (it’s used in Barbaric Bracers I believe XXXX), Iridescent Pearl, Small Lustrous Pearl, Black Pearl & Shadowcat Hide (used in Shadowcat gloves? XXXX), Elemental Fire, Essence of Fire, and Mithril Bars. I usually just type in pearl to scan for all the pearls available, and I click the Elemental and Essence items I have, then delete the “Fire” part. This allows me to scan for all the essences and elemental items on the market, and bid on the ones that have low starting bids. I’m not high enough to be able to kill the monsters that drop these items, or survive in the areas where I can mine some of these gems, but I know how to take advantage of optimistic people who put items in with a low starting bid. :)
I also try to remember to check for Mithril, because it’ll come up with mithril ores and bars, along with Mithril filigree (JC) and Mithril items from Blacksmithing. If you have a Miner that you’ve powerleveled to be able to smelt Mithril, you can take advantage of the price difference that sometimes happens between Ores and Bars.
If you have a Jewelcrafter, you’ll know that it takes 3 Mithril Bars to make a Mithril Filigree, and it’s annoying to make them if you’re not getting skill points from it. It’s often posted at a very cheap price, so if you start looking for it early, you won’t be wasting mats that you could sell or use on other professions.
If you have an enchanter, know that you can often find good items to disenchant on the AH by typing in Mithril and Mageweave. Items that have those keywords are a big part of Tailoring and Blacksmithing, so people will post a lot of them at once, with a low bid price.
When buying recipes for alts, be aware that some green recipes are actually vendor sold, not drops. Cat’s eye goggles (stealth detection) are frequent rewards from renewing your Gnomish engineering membership card, so I wouldn’t bother getting them off the AH either, unless it’s a really low bid of like 15 silver, or a buyout of 50 silver. I renewed 5x one day, and the next day received 2 of the Cat’s Eye goggle recipes as gifts for renewing. But, the 3rd gift turned out to be the recipe for Lil’ Smoky (robot non-attack pet), so I was pleased.
Like I said, don’t waste your gold on vendor/trainer sold recipes that you see on the Auction House. Send an alt there to camp, and every so often check back in to see if you can buy some recipes. Since you’re there, you might as well make it profitable, so buy as many recipes as you can afford. I’ll point to Azshara’s southern coast lighthouse for the Deepdive Helmet recipe (36 silver?) that you can easily sell for 3-4 gold. The other vendor in that building has the Blue Dragonscale Breastplate recipe on occasion (sell for 20-25 or even 30-40 gold), and Chimeric Gloves (sell for 7-9 gold) more frequently.
The Hinterlands also has a engineering supplier that sells the Mithril Mechanical Dragonling plans (40 silver???) that can sell for 5-10 gold, as well as a vendor outside that VERY occasionally sells the Ironfeather shoulders recipe for 1.6 gold. The Ironfeather Shoulder recipe can easily sell for 20-25 gold on the Auction house.
The Mechanical Dragonling recipe is sold by a vendor in STV, as well as the Deadly Scope recipe.
Mithril Scale bracers are a Blacksmithing recipe that is rarely sold on the Auction house, that are obtained in the Hinterlands Ally camp, and the Horde camp in Swamp of Sorrows. If you’re Ally, the Herbalism Gloves (+5 herbalism, SW Ashenvale) and the Red Whelp gloves (chance on hit for fiery damage, Wetlands) are cheap recipes that will sell well on the neutral auction house, because they both are sold by Ally vendors ONLY.