Give to all your Items a value, 1 = Sword, 2 = Shield and so on. Use a skill for that. Now if you pick up a Item a global var becomes this value. The Inventory have for every Slot a button. Create also a var array with the size of your Slots and a bmap array with the Bitmap of your Items. If you click to this button, change the Slotarray to the value of the global var and the global var to the Slotarray. Now you can change the Bitmap of the Button with pan_setbmap().
That is how a simple Inventory works. For more complex Inventory i use a Structarray for the Items and one for every Slot.
