Alchemy, at least once you have reached a certain level of skill and acquired a few high-demand recipes, is a fairly good money-making venture (especially when combined with Herbalism).
Compared to other production professions (Blacksmithing, Enchanting, Leatherworking, Tailoring), goods you sell from Alchemy are, for the most part, consumable so your customers will keep coming back for more. Also, there isn't anything that requires farming ridiculous amounts of rare materials; even the highest-level recipes require only a few herbs per potion. Although some of the recipes are quite difficult or time-consuming to obtain, these recipes will become your premier money-makers.
As with any economy the main way to make money involves supply and demand. Be prepared to spend top price for rare recipes and spend hours grinding reputation with several factions. If you have Herbalism as well as Alchemy, be sure to check how much you can just sell the raw materials for. For the common recipes you can often make more selling the herbs than you could by turning them into potions!
Before making potions to sell, check prices and demand on your server. You should be able to make a profit from buying herbs from the Auction House and selling the finished product. If you cannot make a profit from doing this at higher alchemy levels, then you need to find better recipes.
High Demand Products
Healing and Mana potions are constantly in demand, although you'll find more buyers for the higher-level versions.
Especially the highest-level versions and the ones that buff multiple stats, such as Elixirs of Strength, Elixirs of Intellect, and Elixirs of Major Agility are usually good sellers. Elixirs of Damage boost spell damage, like or , and are also often in demand. Again, the higher-level versions will sell better, since higher-level characters have more disposable income.
Quests and Professions
Some potions are needed for quests, or for other professions. are needed to make (an excellent set of blue gloves); there is a quest that needs a Strong Troll's Blood Potion; is needed for a number of Engineering recipes; is needed for a number of Tailoring recipes; is needed for the Warlock epic mount quest. , in addition to their PvP demand, can be used to make and . and are also used in various recipes or weapon enchants. Transmutation is also quite valuable, especially high-end transmutes involving primals. The Thorium->Arcanite transmute (since it is needed for so many other professions' pre-TBC top items) is fairly useful, but not as it used to be. Prices have also gone down sharply after the 23 hour cooldown was removed in patch 2.4 (was 48 hours before TBC). Mithril->Truesilver can produce some decent (but not spectacular) returns from level 20 until level 35.
Depending on your server, the greatest possible profits may be in making potions that are useful for high-level characters running endgame instances — For instance, in the past Greater Fire Protection Potions were in high demand for running Molten Core, and AQ content was driving up demand for Greater Nature Protection Potions. The outstanding buffs provided by the "Flask" recipes are also always in demand. With the newly updated requirements for resistance Flasks in some Outland instances, and profession specializations, Flasks are quickly becoming the raid consumable of choice (their ability to persist through death is highly useful).
As of Patch 3.0.8 Flasks no longer require an alchemy lab present to create. Such labs are found in Scholomance, Blackwing Lair, Shattrath City, and Acherus: The Ebon Hold. Shattrath may only be used if you have the Burning Crusade expansion, and Acherus: The Ebon Hold may only be used if you have the Wrath of the Lich King expansion. Scholomance and Blackwing Lair require fighting in order to reach the labs, while Shattrath City and Ebon Hold are sanctuaries.
Low Supply Products
Due to hard to get ingredients
Herbalism will allow you to gather most of the materials yourself. This works even at lower levels, there are a number of potions at the 225 skill level or below that are still useful to higher-level characters, but farming the ingredients for them is often a slow process, and a high-level character will gain little of value other than the herbs. Fishing can also be useful. Alchemy-related fish (, , and especially ) are very valuable relative to the time taken getting them.
If you have Herbalism profession, consider farming . It is one of the rarest herbs of the game, as it is found only in graveyards (and only at a few of the many in the game). Grave Moss is an ingredient for both and . The Kodo Graveyard in Desolace can be a good source for Grave Moss. Another valuable herb is . This is the rarest of all herbs in Azeroth (not including those exclusive to instances)and is prized for its use as an ingredient in Invulnerability Potions.
At level 70, many of the best-selling potions and elixirs are those that require Nodes of which are exclusively found in instances introduced in The Burning Crusade. and both sell for high prices but require either running high level dungeons or farming the Ancient Lichen off various mobs, at a very low drop rate.
Due to hard to get recipes
Flasks used to be a fantastic source of income for any alchemist lucky enough to have one or more recipes, mats (especially ), and regular access to an Alchemy Lab. Since the release of The Burning Crusade and the availability of most flasks through reputation grinds, and later the relaxation of the discovery system, all three of these have become much less hard to find, and flasks have been reduced in power as well, reducing demand. However, they are still limited in supply on many realms and remain popular for heroic instances and raids, so it is possible for a flask-making alchemist to find regular buyers.
Keep an eye out in the AH for any Alchemy recipes selling cheaply. This is less common than it used to be, but sometimes people will sell recipes for considerably less than their market value (since most of them have misleadingly low vendor sell values). *Most* lower-level recipes that are not trainable can be bought from a vendor in one of the cities (though some are limited supply, so you may have to camp the vendor for a while to get them). But others, like the , are random drops and are very difficult to find.
Also keep an eye out for anybody selling herbs for low prices. Buy low and sell high, or hang on to herbs for later recipes.
Make as many transmutes as possible, even if you have to spend your last money on materials. It is easier to sell transmuted materials in stacks of 5 or more. Of course you can sell your transmutes only, but will not profit as much as by transmuting for yourself and then selling.