Do your research
Don't just pick a profession because you heard somewhere that it's good for making money. Figure out what you will buy, what you will make and how much it sells for. Every single market I've written about has its own profile, and you need to focus on the ones that will do the most for you for the least effort.
Let's say you're investigating the cooking market. Use Market Watcher (guide here) or something like it to keep track of the price of cooked food, raw ingredients and Northern Spices for a week or so. Calculate your margin, make a guesstimate about the volume you could expect, and see how long it would take to pay off leveling cooking.
The law of diminishing returns
It's right up there with conservation of energy. There is a critical point in every market you enter where the more time you spend, the less profit you get out of it. The only solution is to diversify, because then you can cherry-pick the most profitable parts of every profession and focus on those. Let's say there's a market for glyphs on your server that takes you an hour a day on average to serve; will you make more money by introducing another cancel/relist cycle daily, costing you another half hour a day plus all the additional crafting time, or would you make more money by spending that time getting into tailoring? Heck, what if you spent that extra time mining? Farming is never much money per hour (and it's way less fun for me), but knowing how much you can make per hour circling Sholazar will tell you when you're just spinning your wheels.
Of course, cherry-picking the most profitable parts of every profession means you have to have more characters and more professions. That's a significant up-front investment that will need to be paid off before you can even count the profits you're getting.
The only good death knight
The limitation on primary professions is part of the barrier to entry to creating a crafting empire. You can get around it once, but after that you're going to have to do all the work. Assuming you have a main at 80 already, you have the ability to train two professions off the bat, and unless your main is a DK, you have the ability to faceroll a baby DK from level 55 to 65 (the minimum level for 450 professions) without much work. Even if your main is a DK, you still have a level 55 or greater character somewhere that you had to level to unlock DK creation (assuming it's not deleted and it's still on your main server). Every two profession slots after this will require you to get to 65 the long way.
Bear in mind that certain markets are unavailable for level 65 characters, even if they have 450 professions. I can't go making the Mongoose enchant on my 65 death knight because the formula drops off someone I'll never see (well, not until I start leveling my profession alts for Cataclysm, at least).
Additionally, I am involved in a few professions like jewelcrafting where I make much more money off the lower-level markets (prospecting old, lower-level ore) than I do off the Northrend-level markets. If I had needed to limit my leveling time and spending on leveling professions, this could have been a place to do it.
If you plan on being a fairly passive auctioneer, you will want to pick professions that trade scalability for risk. For example, jewelcrafters have the ability to do a very profitable daily, alchemists have the ability to transmute epic gems and tailors have bag cooldowns. You can also trade off between profitability and scalability. Alchemists can AFK transmute titanium any day they can buy the mats for less than they can sell the product.
If you want the ability to scale up and don't mind a bit of risk or elbow-grease, you can graduate to inscription, enchanting scrolls and mats, selling cut gems, making flasks or potions and selling lower-level bags.
Whether you have 3,000g or 300,000g to your name, spend a few minutes navel-gazing and see whether there's a change you could make to your profession roster that could make you more efficient.
Being an auctioneer is like being able to print money (or gold, as it were). Wait, that doesn't make sense ... You can print on gold, but you can't print gold. That would be closer to transmutation? I can transmute titanium, but that's only worth it if the price of saronite is low enough to justify the time spent making it. I need some sort of analogy here. ... Whatever, I'll figure it out later. Making gold? Every week, Gold Capped will teach you the tricks of the trade, from setting up your auction addons and user interface, to cross-faction arbitrage, to learning how to use your trade skills.