I would use as many instance variables to objects as possible.
The names can be whatever you want ...
example
sprite Slots
slots.active = false
slots.slotnum = 1
slots.item = url
slots.active = false
slots.slotnum = 2
slots.item = url
event
on click slots
sub
slots.active false (inverted)
action
set slots.active true
sub (to slots active false)
slots.slotnum = 1
create button on x,y
sub (to slots active false)
slots.slotnum = 2
create button on x,y