The choice between the three specialization seems obvious, if you use lot of potion, you shall be potion specialist. Well, the goblin saying says "seems" is the opposite of "is". Why? Potionmaking takes a second, therefore in an unlimited supply. Theoretically one potion alchemist can create potions for the whole realm. Practically he can serve a guild or customers from the trade chat. You can always announce "LF potion spec alchemist to create X, I tip 5G, I get all the extra proc". In larger quantities it worth to make such announcement, even if you are an alchemist too. Elixirs can be made in unlimited amount too. Therefore potions and elixirs are rarely more expensive in AH than their materials. Usually they are cheaper, thanks to the alchemists making skillups.
Transmutation on the other hand have a cooldown. It means you can only make 1 transmutation in a day. This means limited supply. This means profit. By performing transmutation you can make profit without the extra proc, so it's more than suggested to all alchemists. On Kul Tiras realm the primal prices:
- Fire: 34G
- Mana: 30G
- Earth: 5G
- Life: 16G
- Air: 36G
- Water: 30G
- Shadow: 26G
- Might: 131G
But let's see the real diamond: earthstorm diamond. The materials cost 106G, the diamond cost 142G. It's 36G profit. But since the extra proc scales with price, if you are transmute master, your profit is 60G.
On other realms maybe Primal might or Skyfire diamond is the best seller, but one of the three, considering the 17% extra will surely make you nice profit. Every day, without risk. Not by sharking, but by working, by supplying when other supplies are limited, since other alchemists are also on cooldown. Fair trade.