Buying and reselling zz'rot for extra item.

I don't know if this is a bug or something but I saw someone buying a zz'rot, placing it down then goes back to base and sells it to buy another item, but the one he already placed remains there. So essentially he gets an extra item. Every time his zz'rot is destroyed he just repeats it. Sell item, buy zz'rot, place down, sell, rebuy original item.