Currently, a category for which you once had items will show up in your trade screens even when you’ve since sold or used up all the items in it, resulting in an empty category showing up in the screen. I don’t think that should be the case, probably. See attached video at 43:55.
(Note that it happens to be the uncategorized category—I made some shields just to sell them because they make good money—but that’s just happenstance and is not what this topic is about.)