In trade screens you can view your own character's inventory or select any of your followers to trade items from their inventories. However the mouse-over compare tool always compares the seller's items to what your character has equipped.
I think it would be better if the compare tool compared to the equipped items of whichever character you had selected on the trade screen. After all, if I'm buying something for a follower I would rather see that the follower has the proficiency for that item, and that it's actually better than the item they already have equipped.
