So let's say you were some financial market that operated solely in a US state so all of your transactions never crossed state lines. Would federal government agencies like the Commodities Future Trading Commission be able to regulate you? As far as I understand it, it shouldn't because it's not interstate commerce, right?
Imagine, for instance, that you're a dairy and poultry farmer in Ohio. You grow a small crop of wheat every year to feed your animals. From whatever is leftover, you take enough to make flour for yourself and your family. After that, you sell whatever is left to someone locally.
You could argue that your wheat-related activities would not be interstate, that they would not be commerce, and that they would have at most indirect effects on interstate commerce.
But the Supreme Court would reject that argument, as it did in Wickard v. Filburn, 317 U.S. 111 (1942).
Even if appellee's activity be local, and though it may not be regarded as commerce, it may still, whatever its nature, be reached by Congress if it exerts a substantial economic effect on interstate commerce, and this irrespective of whether such effect is what might at some earlier time have been defined as "direct" or "indirect."
So the general rule is that any activity that has a "substantial economic effect" on interstate commerce -- by reducing demand, for instance -- is within the reach of the federal government's authority to regulate.
While there continues to be controversy over this broad interpretation of Commerce Clause authority, it's a well-settled principle, and everyone recognizes that practically speaking, there's virtually no commercial activity that Congress cannot regulate.