If they weren't selling, I'd agree with you. However, I do not have any problems selling Spellthreads regularly. My entire stock is gone the same day I craft it. Even if I had to repost them, it only costs 41s to post 1 for 12 hours. That is so insignificant it's laughable.
There is lots of Royal Satchel competition on my server, so it's not uncommon to have to repost them several times before they sell. I started out selling bags, but became too frustrated that I switched to Spellthreads, and ended up using all the bags I crafted myself.
Overall, I gain more profit, sell more consistently, and am less frustrated with Spellthreads.