It's actually just an array of items Then you need a panel to show the items, with a click-event to swap the clicked item with the item held by the mouse.

Of course it way more complicated if you e.g. want to create a "tetris-inventory" (diablo-style inventory). It's as well a bit more complex to re-write an inventory system for a system like this (RPG Template), as it has to fit into the other code, and due to that fact the original item system worked in a different way.