How many times have you gone to the store, and bought something that you already had plenty of at home? Recently I bought a gallon-sized container of all-purpose cleaner at Costco, only to find the other gallon-sized container of all-purpose cleaner I had bought a few months back tucked underneath the sink.
Darn it. 🙁
Had I had a “do not buy” list in place like My Broken Coin had implemented, I would have saved myself having to store two gallons of cleaning fluid. That certainly would have been on my list. We're good about keeping track of what we need, but not as good about what we don't need.
I think the trick of this method is to review the list regularly so that when the need arises to take something off of the “do not buy” list, it's taken off.