Hud question: Why cant I move items to different slots in the store window?

I keep trying to do this in game before realizing it doesn't work, the store window has a grid with your current items that matches the normal hud grid but stays the same size even if you scale down the hud, so it would be much easier and more convenient to be able to move them from here while buying than needing to close the shop and move items around while getting to lane using a smaller part of the interface