Upon entering the gallery, find your way to the historic Etherington house.
The closet that has been made available for transactions by Shadow Librarians + Phantom Merchants can be found underneath the staircase:
Open the door to the closet.
Enter the closet.
In the closet, you will see four slots for Clipboard Drop-off on the floor. Select a vacant slot to deposit your clipboard.
The image below depicts a Dropped-off Clipboard, deposited successfully into a vacant slot.
An artist rendering.
Leave the closet and close the door behind you. Sometimes you need to give it a bit of a pull as there is a cable running underneath the door.