To store items, you need to stand next to a shared stash, right-click on an item in the inventory (
stash. To access the stash, press
U to open the stash UI while standing next to it. To move items from the stash to inventory, right click on an item and press
unstash in the stash UI.
The stash UI can be scrolled down with the mouse wheel. Currently, there are no limits to the number of items you can store in the stash.
Items in the two stash are linked and shared by all characters in the same account, so you can use it to transfer items to other characters.