Look up potions in the item database on this site. Take a look at the right hand side where there is sometimes an entry like this:
Used in Quests:
Coolant Heads Prevail
A Broken Trap
The example above is for Frost Oil. Those potions that are used in quests will always have at least a trickle of demand. In the case of Frost Oil, one is a levelling quest in the Badlands (need 1 potion per character, Alliance side) but the other is used in Dire Maul 'Tribute' runs and therefore constant demand.
The same applies to some extent for those used in crafting.
BTW if you don't pick your own potions don't expect to make much profit from just brewing pots. Most herbs are priced at or above the value of the potions you make from them.
Also you have an excellent slowburn money-maker in Transmutes. Arcanite as well as elemental Essences are in high demand for crafting, especially Essence of Air which is used in popular DPS class enchants. You can often buy Essence of Water, transmute to Essence of Air and sell at good profit. However there's a significant cooldown time so it's not a license to print money - just a good steady flow every day.