These are the steps (reproducable for me at least):
- Select any item on page 1 of the house merchant
- Click with the item selected anywhere on the housemerchant page (but not the little circles or you'll drop it)
- this makes it the item is stil on your cursor, but not really selected anymore
- now click an empty slot in your bags
- this will drop the item that is is in the last slot of your personal vault in your inventory
https://youtu.be/0rcoy_I8dxw