Alchemy is actually a little bit of an odd critter in terms of understanding their end-game crafting materials. That's because Alchemists not only create important final products for raiding, but also because they transmute gems for Jewelcrafting
The Transmutes are incredibly valuable, since Alchemists will be able to upgrade blue-quality gems into the sought after epic-quality gems like the Cardinal Ruby. For this reason, most Alchemists will be consistently buying up (or creating, if they can) the blue gems used for those transmutes. The elemental pieces like Eternal Life also go into this transmute process, so you'll often see Alchemist farming those from Revenants.
Raiding flasks are the other big produce you get from Alchemists, and they all use either Lichbloom or Icethorn alongside some Frost Lotus. All three of these herbs, obviously, have sold for large sums on the Auction House. Nowadays, however, Frost Lotus isn't quite as rare as it used to be, so you should find these herbs a little more accessible.
The basic ores of Blacksmithing are Cobalt and Saronite, of course, but the real end-game material is Titansteel. Titansteel is created from three Titanium bars, an Eternal Fire, an Eternal Earth, and an Eternal Shadow. For most of the expansion, you could only create one Titansteel each day. In upcoming patch 3.3.3, however, that cooldown is being removed. Titansteel can only be created by character with the Mining profession, since Smelt Titansteel is an ability granted by Mining.
Blacksmithing, like most of the crafting professions, can blow through an immense number of Eternals. Fire, Earth, and Shadow are probably the most common Blacksmithing elements, especially given the profession's high reliance on Titansteel.
Enchanting is one of the few professions that is not only a crafting profession, but also its own gathering profession. However, it's also the only gathering profession whose produce gets automatically rolled on by the entire party, thanks to the Dungeon Finder tool.
The four most important materials for Enchanting are Greater Cosmic Essences, Abyss Crystals, Dream Shards, and Infinite Dust. Abyss Crystals and Dream Shards are fairly easy to come by, though. Even if every one in your group does roll on these disenchanted items, it's easy enough to queue up for another Dungeon until you eventually have enough. Infinite Dust tends to be a little harder to get together, because it seems like so many end game recipes use so darn much of it.
As is the case for so many professions discussions, Engineering is totally an odd duck. Since most of its creations only need constructed once, the most commonly created Engineering items are the Icecrown arrows and bullets. These only use two Crystallized Shadow or Crystallized Earth, allowing the engineer to make plenty of the ammunition in their free time.
Inscription's unique snowflake reagent system actually boils down to herbs. The scribe buys herbs, which they then grind into various inks. Ethereal Ink, from Nether Pigment, can originate from just about any Outland herb. It's a little random what pigments Inscription master receive. For this reason, you'll find scribes buying up huge quantities of low-priced herbs from the Auction House, milling it all out, and then creating their produce from whatever they happen to score. Mostly, those who practice Inscription like herbs. All herbs. (Except Frost Lotus, which doesn't directly produce anything.)
Jewelcrafters are the chief consumers and creators of gems. However, there's not really "one or two" gems in particular used by the profession. The only real factor in deciding the most popular gem is "what gets purchased the most on the Auction House." Some folks swear that the Cardinal Ruby moves better than any other gem, but I've had the most success mixing up which gems I offer.
Jewelcrafters, however, will crave the raw titanium ore, which they can Prospect into high-quality gems.
Leatherworking is almost never taken without its sister-profession Skinning. For that reason, most leatherworkers who need materials can be found out in the field, farming animals for their flesh. However, Nerubian Chitin, Jormungar Scales, and Icy Dragonscales are particular leather types that must be farmed from specific mobs. For that reason, you'll sometimes see leatherworkers cruising auctions or the Trade channel to fill a specific need.
Leatherworkers used to care a lot about the rare-drop Arctic Fur, but it can now easily be purchased in Dalaran for a mere ten Heavy Borean Leather.
Tailoring is getting some cooldown changes in the next patch, which is going to radically shift their economy. Whereas specific tailoring specialty items like Moonshroud were at a very high premium, these items will vastly drop in scarcity when the creation cooldown is removed. That makes skilling up Tailoring through the final levels much, much easier. It will also, mean, however, that tailoring will return to caring about raw Frostweave, which can be upgraded to the final Moonshroud materials. (Or, other specialty material.)
Good luck, good profit.
(Edited to avoid some confusion, as I had used a bit of slang that would probably mislead newer players.)
Each week, Insider Trader takes you behind the scenes of the bustling sub-culture of professional craftsmen, examining the profitable, the tragically lacking, and the methods behind the madness.