However, the very basic reason to be a particular profession usually has to do with what you can sell in the Auction House. The character bonuses are certainly a huge perk of training a profession, and shouldn't be ignored by any player looking to cruise endgame content. But the driving reason for many people to pursue these skills had to do with other items which produce cash. Cold, hard, beautiful cash.
Let's jump behind the cut and talk about the saleable items from each profession. While you might choose your profession based on its character bonuses, it's still best to know what items will sell on the AH from each.
Gathering Professions: Herbalism, Mining, Skinning
All three gathering professions are very powerful for producing saleable product for the Auction House. Selling the raw materials gathered with these skills will fuel the items required for raiders to punch down Icecrown Citadel, or even casual players who want to make sure their characters are at the pinnacle of their personal progression.
Herbalism's big-sell item is the Frost Lotus. The Frost Lotus is the cornerstone reagent for creating Flasks of Endless Rage and Flasks of the Frost Wyrm, two of the most popular items on the Auction House. Even Herbalist/Alchemists will probably buy the occasional Frost Lotus stacks if they find themselves without time to farm. Your Frost Lotus should sell fairly quickly from the Auction House, assuming you've got them reasonably priced. The down side of the Frost Lotus is that it drops at random from any Northrend herb. It's hard to farm these on purpose.
The other benefit to Herbalism is that there are two crafting professions which are fed by the skill. Both Alchemists and practitioners of Inscription will need herbs to produce their final product. You'll not only pick up cash from people who need flasks, but you'll also sell to people who are trying to advance these skills.
Skinning has always been a reliable gathering skill. If you have some time and the ability to kill mobs, you have the ability to farm leather. It doesn't feel like there's quite as many skinnable beasts cruising Northrend endgame as in Burning Crusade, but it's not all that hard to find lower level skinning opportunities. Skinners also have a few more varieties of leather to collect in Northrend, like the Nerubian Chitin and Jormungar Scales. This variety will probably lead to an ebb and flow in individual cost on your local economy, but it basically means that if someone needs one kind of skinning produce, no other type will do.
Like Herbalism, Skinning has a cornerstone product. The Arctic Fur is used in many precious recipes and is used to actually purchase certain Northrend recipes. Both extant and leveling leatherworkers will need the Arctic Fur, and that creates a constant demand for the material. Like the Frost Lotus, however, you can't specifically farm for Arctic Fur -- it has a random chance to drop from critters you skin. The final real down side for Skinning is that it mostly only fuels Leatherworking, so you're not going to get a lot of cross-traffic in your product.
Mining is the final gathering profession, and it's arguably the best of the three for picking up huge amounts of cash. (We should acknowledge right off the bat that the argument depends on your local economy.) This is because the produce from mining is used in three different professions: Blacksmithing, Engineering, and Jewelcrafting.
With three different crafting professions desperate to buy metal ore, you'll almost never lack for customers. Since the ore creates many of the important gems that players will constantly be replacing with each new gear upgrade, the stuff is indispensable to jewelcrafters. Blacksmiths use smelted bars to create new gem sockets for themselves and their customers. Ultimately, you really just can't go wrong selling ore on the auction house.
More importantly, miners have an important cooldown. They are the only profession that can create Titansteel, by smelting three eternal elements into some titanium. Titantsteel usually sells on the auction house for a hundred gold over the sum of its parts. That's because the miner isn't really selling the metal, per se. They're selling the use of their cooldown -- you can only create Titansteel once every 20 hours.
Even though the gathering professions are usually the most reliable money makers, every profession has something they can sell. Let's go through the crafting professions, now, and look at the most viable products from each.
We've touched on Alchemist's flasks, but those form the key structure for how Alchemists make money on the Auction House. Flasks of Endless Rage provide a long-term attack power buff, and are sought after by any physical DPSer looking to cruise raids. Flasks of the Frost Wyrm are the spellpower equivalent, which will be sought out by both magical DPS and healers. If a character is hurting for mana, then the Alchemist can provide the Flask of Pure Mojo. The tank flask is the Flask of Stoneblood, giving a valuable bump to hit points.
While many blacksmithing products were valuable at the opening of the expansion, most of the weapons and gear are easily leap-frogged by emblem gear. This regrettable condition has left only the Eternal Belt Buckle as the blacksmith's viable product. Sure, you could still put up a Titansteel Destroyer, but you'll probably make better cash selling the materials themselves.
Enchanting is the first of two ultimate selling professions. Most new gear will need an Enchanter to give it that extra stat boost, so you'll constantly be in demand. Even better, Enchanting doubles as a pseudo-gathering profession, since you can cycle unneeded drops into raw materials. Those raw materials can then be sold on the Auction House. You might want to make friends with your local Inscriptionist, though, to obtain the vellum needed to be able to sell enchants on the AH. Still, most Enchanters I know are comfortable pitching their services in their faction's trade channel.
If you've got appropriate amount of Ashen Verdict reputation, you can sell arrows and bullets. As long as there are hunters in Wrath, you've got cash flow.
Just about everything you make from Inscription will sell on the Auction House. However, it probably won't sell for very much. This is because people don't swap their glyphs all that often, so they only to need to buy so many. However, Darkmoon Card: Greatness is still worth huge amounts of cash. It's the best trinket many players will ever see, and they will pay for it appropriately.
Similar to Blacksmithing, there were a lot of viable products for leatherworkers at the opening of Wrath. Since then, however, they've really been mostly restricted to leg kits. The Icescale Leg Armor is the kit for your physical DPS, and will still fetch a healthy price on the AH. The Frosthide Leg Armor is for tanks.
Jewelcrafting is the other of two ultimate selling professions. Every single time a WoW player picks up a new gear item with gem sockets, they'll have to replace all of the old gems. As long as there's new gear being made available to players, those players will need gems. You usually want to focus on DPS gems, since there are 15 DPSers for every 7 healers for every 2 tanks. (That's just a rough estimate -- don't forget the dozens of DPS players who never get to see the inside of a raid.) However, tanks and healers will need their own gems also.
Tailors used to be able to sell bags and gear on the Auction House, but that's slowed down this late into the expansion. However, tailors still have magic-focused leg kits as a viable products. Brilliant Spellthread and Sapphire Spellthread fly off the shelves as quickly as I can get them on the AH. Also, since it has one of the longest cooldowns in the game, tailors can sell Moonshroud for a tidy sum. After all, you can only make it about once every four days.
Ultimately, if you choose your profession solely to make money, you're probably best off sticking with the gathering professions. Since that produce fuels both leveling crafters and existing crafters, your source of customers is much more guaranteed. Still, every profession has something it can sell, so they should all be worthwhile.
Now go make some money!