Is there a way to unstack inventory items? I can't figure it out. I have a few robes from one of the bosses in the spring fest, and I would sort of like to be able to trade one at a time and not all of them, heh

Yup! It's probably the most unintuitive thing on this site.
Hold shift while clicking on an item. You can then release shift. Then click on any empty spot in your inventory.

There is a note about how to do this right below "switch to equipment"