You can buy the craftable items from anyone who crafts. It depends on their level as to what you can buy. If they can make it, then you can buy it as long as they're not part of the quest. So if you have a low-level physician, then you can only buy the weak salves and tonics. It doesn't matter if they have them in their actual inventory. I can buy things from my spy, for example, that I know he has never actually made.