This idea is the easiest one to do. Simply get a storage unit and start buying materials as you see them on sale. If you have a garage where you are at, this works just as well. The point is to take advantage of every sale out there.
As we all know, sales don't last and you need to be ready to scoop up the bargain as you see it. For instance, sometimes Costco will have toilets. Very nice toilets I might add. But they don't carry them all year or at every location. When you see them, buy all that need and wait until the plumber needs them to install.