When setting up a stall, you can stack the SAME items in one slot rather than in different ones to save your stall space. To do that, press the Ctrl button, left click on the overlapped items in your inventory, and drag the same items into the same slot in your stall sheet.

lol, try it, hope you will like it. Lots of info. on PDO's official website, i just copy some to share with the players who enjoy playing PDO.