Yag necklaces sell because

1.) Wind + Necklace = 3 grass threads and HQ can make a stack(<3!)

2.) 4 Yag necklaces can earn you 200gil if you give it to the cat burgular. This quest not only gives you gil, but Tenshodo fame which you need to turn it approximately 275ish(it varies but i last saw this one) to get the necessary scroll of Utsusemi Ichi. That and Tenshodo fame quests are rare as hell. Theres only a handful that work and this necklace one is the least painful on your wallet assuming you farm them.

I personally dont recommend you selling yag necklaces without getting Utsusemi first. @_@; Even then i would say...Grass Thread for the win.