I've noticed that when I find a Scriptorium or Chancery in Explore and decide to swap out a recipe or invite, sometimes most of my available items for trade will be grayed out, and I can only trade certain ones. It doesn't seem to be limited to similar type (e.g., only pet magician invites, only golem blueprints, etc.), but rather the selection seems to be random. Also, it doesn't happen every time. Why is this?

Oh, never mind, figured it out.