I'll start this post by saying that Faid over at Clockwork Riot has a great list of vendor recipes for sale for $2. It's well-edited, nicely laid out, and contains some extra tips. If you're brand new to selling vendor recipes, I recommend picking it up; my post is going to be a little less straightforward and assumes a bit more knowledge of TSM and basic AH techniques. Please note that when I say "recipes" here I mean any profession recipe/pattern/plan/schematic/formula/whatever, not just cooking recipes.
So, vendor recipes. I've had a lot of luck selling these, to people who are either too lazy to go pick them up from the vendor or who don't know they come from a vendor in the first place. Their ignorance is my bliss. I did a vendor run of half of EK on my mage yesterday and pulled in about 1200g. It's never going to be your primary money maker, but every copper counts, right? Right. How do you get started selling them, though?
1. Know where to get the recipes in the first place.
As I mentioned above, Faid has a nicely edited guide that lists profitable recipes by location in an orderly fashion. That's going to be the easiest way to find them -- she's basically handing the information to you on a plate.
You could also look on Wowhead and try to sort out everything and make up a big huge list and so on and so forth, but let me tell you: That would be a giant pain in the rear. Seriously.
Or you could do what I mostly do, and get an addon called RecipeProfit for GatherMate. Obviously you need to have GatherMate 2 installed in the first place, but you probably already do. If you don't, you should. RecipeProfit adds markers in for vendors and when you mouse over the marker, it shows the vendor's name, the items they sell, and how many you have. That last feature is limited, as it only counts what you have in your current toon's bags; it doesn't make use of Altaholic to show your other toons or auctions. Still, lots of information. It shows the icon both on the minimap and on your big map. Here's what it looks like:
1b. Make a flight plan.
No matter where you get your information, buying up the vendor recipes is going to involve some serious travel time. Many of the recipes are limited supply so you'll only be able to buy one at a time, but they're not all going to sell out on the same day so if you do this once a week or every few days you can build a slight stockpile. (I created a bank toon specifically for vendor recipe storage and selling, because I get confused when I post too many categories of stuff on one toon.)
Originally I had planned to grab recipes on my mage, since she can teleport around easily. However, I've ended up mostly doing it on my hunter, who has archaeology. I combine recipe buying with rare spawn hunting and archaeology dig sites. That way I'm not just flying around buying stuff. This would also pair well with a gathering profession.
After you've done this run a few times, you'll come up with an idea of how to get to the vendors most efficiently. Again, Faid's list is already organized for you, so if you're overwhelmed by the idea, go buy that.
2. Sell the recipes.
I use TSM for this, because individually listing each recipe would take forever, given that you can have hundreds of different items up at one time. I created an auctioning category in TSM just for vendor recipes, and I have several groups within that category for different price brackets. The vendor recipes sell for very different prices, so I didn't want to just throw them all in with one threshold and fallback. The threshold/fallback spreads for each category go from just above the previous group to the price the group is named after. That way I can undercut. You can undercut on these pretty seriously since you'll rarely pay even as much as 5g for one of the recipes. Then you just go to the auction house, hit Post Auctions in the TSM tab, and wait for the gold to roll in.
2b. Stay organized.
As I mentioned earlier, I have a bank toon specifically for vendor recipes. I set up TSM auto mailing to send all the recipes to her, so I don't have to pick through my bags; it saves on clicking and mailing time. I recommend doing this. Just create the toon as a mail-to character and then add all your vendor recipe groups. Voila, done. I also use AdiBags, which separates the recipes by profession automatically. You don't really need to do this since you're going to be selling everything, but it's there if you want it.
So that's it -- vendor recipe selling in a nutshell. Good luck!