There is no 'marketplace' for ideas because ideas don't cost anything. No one buys and sells ideas - except in the limited commercial setting in which IP is sometimes bought and sold.

You can agree or disagree with an idea at no direct personal cost. You can persuade or dissuade others at no direct personal cost.

So how is this a market?

And as Poemless pointed out, there is a huge difference between commercial advertising, which has more in common with the pro-party posters and slogans you'd find in the old Soviet states (only the people are better dressed and the colours are brighter), and public debate and discussion, which is based on persuading opponents by engaging with their ideas directly.

As opposed to yelling in their ear with a constant barrage of jingles and catchy video sequences.

a totalising ideology is one that captures all metaphorical space in discourse as well as control of physical structures and processes in facespace.

this is colloquially expressed as "if all you have is a hammer then everything looks like a nail."

I could get into conceptual connections between totalising ideology and monocropping, etc.  -- but perhaps the most succinct subversion is from our old friend physics:  if you try to understand light as particles it doesn't really work, and if you try to understand it as waves it doesn't really work.  both metaphors need to be in play at once, mutually contradictory and mutually necessary.  which is a feature of complex systems...

but we'll be off to Happy Planet of the Verbositoids if I don't slam on the brakes here...

The difference between theory and practise in practise ...

The philosophical core that Justice Oliver Wendell Holmes inserted into the freedom of speech debate over 78 years ago continues to beat like a young heart in the body of First Amendment law. His words were at the center of the U.S. Supreme Court's reasoning when on June 26 in Reno v. American Civil Liberties Union, it struck down the Communications Decency Act as a violation of the Constitution.

In 1919, Justice Holmes filed a dissent in Abrams v. United States in which he created the powerful and enduring "marketplace of ideas" metaphor to encapsulate the concept of freedom of speech. In the marketplace metaphor, ideas compete against one another for acceptance -- with the underlying faith that truth will prevail in such an open encounter.

Borrowing from John Milton's "Areopagitica" (1644) and John Stuart Mill's "On Liberty" (1859), Holmes wrote in his Abrams dissent: "But when men have realized that time has upset many fighting faiths, they may come to believe even more than they believe the very foundations of their own conduct that the ultimate good desired is better reached by free trade in ideas -- that the best test of truth is the power of the thought to get itself accepted in the competition of the market ... . That at any rate is the theory of our Constitution. It is an experiment, as all life is an experiment."

Justice Holmes' pivotal concept gradually became the controlling metaphor in First Amendment jurisprudence. His voice was present in the high court's decision striking down the CDA.