Alchemy consumables are a staple of end-game content and is traditionally one of the best ways to make gold in WoW. So let’s look at what you should focus on in TBC.
Flasks and potions
There are two main types of consumables that are popular in TBC, flasks and potions. Flasks provide a powerful buff for two hours that persists through death. Since you don’t lose it when dying it is generally preferable over elixirs, which would be an alternative. Potions give you strong temporary buffs on a 2 minute cooldown. You can align these with heroism/bloodlust and any on use trinkets or damage increasing cooldowns you have to really burst.
In TBC you can become a master at crafting a certain type of consumable. You can go for transmutation, potions or elixirs (which includes flasks). This gives you a chance to proc extra items when you craft, which averages out to 20% extra over time. This is obviously huge, and choosing the right specialization will help out a lot.
The right specialization depends a bit on what recipes you have, but generally speaking the volume of potions should be much higher than flasks, and in theory be better. The best potion recipes are world drops however, and are quite expensive, so if you cannot afford them then that will diminish the value considerably.
The main way you make gold with consumables is by utilizing a wide variety of stack sizes to help you get a premium price for your items. Players will generally prefer to buy the exact amount they need with one click, and you can and should take advantage of this. I use a setup with a large number of auctioning operations, posting across all available stack sizes from 1-ofs up to 5 of each. With this approach you do not have to be the cheapest auction to get sales, which is a huge advantage.
The majority of alchemy recipes in TBC are learned as discoveries. Whenever you do any TBC craft you have a small chance of making a discovery, learning a new recipe. This means you need to do a huge number of alchemy crafts to unlock every recipe. I would suggest focusing on something cheap you can sell profitably, and then gradually unlocking more as you get more recipes. Super Mana potions are a great item to generate discoveries with, as it’s cheap to craft and sells quickly. It’s up to you if you want to try to sell them for a profit, or if you are willing to take a loss to get more recipes.
There are some notable world drop recipes for Alchemy. These are the Hast Potion, Destruction potion and insane strength potion. All of these are very strong for various DPS classes. Whether or not buying the recipe is worth it depends on the price, and is something you will have to decide for yourself. You can likely sell a large number of these throughout the lifetime of TBC, but the recipes are also quite expensive.
You can find my TSM setup below. It will post according to the strategy I outlined above. As for how much to craft you will have to test things out. It all depends on your realm and competition. Start with 10-15 of each profitable recipe you have, then scale upwards.
Good luck and happy goldmaking!