There are some Job System merchants that do not have the option to sell you their type of goods. According to the
stocks page, they're still keeping track of the items that are sold to them, but they won't allow you to purchase any. For example, I can sell some apples to the Ingredients Merchant, and it will go into the stocks. I can then talk to him or another one and buy apples as well. In contrast, you sell anything to the Book, Gadget, or Weapon/Armorsmith merchants, it goes into their stocks, but you don't have the option to buy goods from them.
I'm hoping all it is is that they need their dialogue updated.