The limits are defined by the market itself.
"the nature of the transaction."
The nature of any particular transaction is irrelevant. The nature of the market is what determines applicability.
""interstate in scope", are truly artificial alterations to the language of the constitutional grant of power."
No. Refer to creation of the shoe monopoly and the various other private actions that provided motivations for the Constitutional Convention itself. Also review the quote from Madison you posted noting that the intent there was to remove State power to favor their own players over those of another State. That intent is contained in the purpose to provide market fairness and equity. That applies to the players, the people. The States were removed from the getgo.
So you say, but there's no support for that view in any of the writings of the Founders. It also doesn't square with the language of the Constitution.
Refer to creation of the shoe monopoly and the various other private actions that provided motivations for the Constitutional Convention itself.
I'd be interested in seeing whatever references you have for this. Certainly no mention of this was made in the Federalist. All motivations for the interstate commerce clause that I've read about (and I've read quite a bit) refer to the actions of state governments, not private parties.
Also review the quote from Madison you posted noting that the intent there was to remove State power to favor their own players over those of another State.
That's not what he said. He was talking about states impeding interstate commerce for their own benefits. He wasn't talking about states getting involved in any other way. But regardless, his point was clear that the motivation for the clause was not to give power (over individuals) to the federal government, but to take it from the states.