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When taking up a non gathering trade, alchemy is one that will always be in demand. Unlike the other crafting trades in World of Warcraft, the items made will always be in demand and scale with level. Some crafting professions allow for the creation of high end wearables but those items can quickly become passé with raiding.
Some professions allow for constant tweaking and changing of stats through stats increases through items and directly on a character's stats. But that is situational on needed when new gear is acquired or a spec toyed with.
But alchemy provides valuable consumables that can be used on the fly despite the 1 minute cool down and limitation of use in combat. From healing, to mana, to faster movement, potions have saved many in varying situations.
Potions are always in demand.
Unlike an enchant on your gear (or permanent alteration to yourself like with inscription), potions are of constant use to a player. Between flasks that give buffs beyond death that last for 2 hours, to potions that when used correctly can be used twice per fight, potions can and do net lots of gold.
Try to post them on the auction house during the hour before your server's peak raid times. The last minute raiders that need them will buy them up net you a good amount of gold.
Items and mats are lucrative on their own.
If you have a bunch of old herbs or alchemy mats you just have no use for, you can simply resell them on the auction house. People who may still need to level their alchemy or who may need those mats for anything else (herbs are not a alchemy exclusive material!) will gladly buy them off of you.
Take note of craft-able items, your cool downs, and alchemy specialization too.
Transmutes and even leveling trinkets are available for alchemists to create. But most craftables like trinkets are bind on pick up. But the bigger money to be made is in not using your transmute cool downs and advertising your profession.
Many people will pay good money, especially to a potion master from the extra potions made, but even more for a transmute master. Since the cool down on an transmute is a good 24 hours+, many will go to transmute master in making gems and bars to save a day and extra mats in making a an item. Getting paid to simply use your cold down is an easy way to make money without using any of your own.