You would be mistaken Destructo. Commerce is a whole lot more than buying and selling, or even shipping for buying and selling things. Interstate Commerce is more than just those items that crossed state lines in one form or another, even previous to its manufacture.
The Miller case in 39 pointed out that even though they didn't sell their short barreled shotgun after crossing the state line, merely crossing the state line put them into interstate commerce.
You could be buck naked, squeaky clean and walking on air one step over the border so you don't even track dirt from one state to another, taking nothing but your body from one state to another and you are in interstate commerce.
Likewise, you could run the biggest chicken ranch in the world, never use anything- not feed, not butchering, nor even the gas in the truck you use to drive to market, nothing from out of state, nor sell your chicken out of state, and you could/would be in interstate commerce because your supply affects the demand for out of state chicken.
Interstate commerce is a fairly broad catch-all that has probably been over-broadly interpreted, but even the Founding Fathers knew it would be, and couldn't find a better way to handle it.