Tailoring gets access to specialized leg enchants that will be popular among casters and healers. They will be relevant for a long time and are selling well already!
There’s a total of four different spellthreads. Two each for healers and damage dealing casters. For each type there is one rare variant and a more powerful epic variant. The recipes are unlocked from the Aldors and Scryers in Shattrath.
The Mystic and Runic Spellthreads come from the Scryers. The Mystic Spellthread requires you to be Honored, and you need Exalted for the Runic variant. They require 5 or 10 Primal mana respectively, as well as 1 rune thread. The Runic Spellthread will also require a Primal Nether, so if you want to craft this one you have to run heroics.
Aldor has the spellthreads for healers. Silver Spellthread is unlocked at Honored, and you get Golden Spellthread at Exalted. They use Primal Life in stead of mana, making them quite a bit cheaper, but otherwise the materials are the same.
Unlocking the recipes
If you do not care about the other rewards I would personally lean towards Scryers as there are usually more DPS than healers. That being said My personal plan is to get both of them. Scryers and Aldor Rep can be brute forced as you get it from quests, and turning in rep tokens that are freely tradeable.
You can also get back into the good graces of the other faction. To do so you need to turn in 1344 of either the Dreadfang Venom Sac for Aldor or Dampscale Basilisk eyes for the Scryers.
Crafting and selling
For the lower quality threads that do not require nethers I would suggest keeping them in stock consistently. They stack so keep as many as you can comfortably afford on your banker. Anywhere from 1 at the time to 5 can make sense depending on your capital.
For the Epic ones your decision will depend on how often you get Nethers. If you run heroics consistently and have a good supply keeping one on the AH makes sense. Otherwise I would look to sell the craft in trade chat primarily. It will also depend on what other Primal Nether crafts you have as you want to sell these with the most profitable recipe you can.
As usual my TSM settings rely on crafting cost as the basis for pricing. If you want to post the Epic quality ones you will have to set a manual price for Primal Nether. You can do so under /tsm crafting – reports, see the screenshot below.
Good luck and happy hunting!