The "marketplace of ideas" meme (along with many of what Americans consider to be First Amendment Free Speech protections) came into jurispridence in a 1919 dissent by Oliver Wendell Holmes in Abrams v. United States in which Holmes defended free speech rights based on the "marketplace of ideas" concept proposed by among others John Stuart Mill. Who was distinctly tardy to the Founding Fathers table.
The concepts are curiously intermingled with much of the reformulation and representation of Adam Smith's ideas which took place over the 20th century, also generally misunderstood by the lay public thanks to an aggressive 60+ year marketing campaign by the Mont Pelerin Society and Atlas Network. But that's another rant.
There's a brief article on the particulars here:
http://scholarship.law.duke.edu/cgi/viewcontent.cgi?article=... (PDF)
There's a longer treatment in Thomas Healy's The Great Dissent.
http://www.worldcat.org/title/great-dissent-how-oliver-wende...
The concepts are curiously intermingled with much of the reformulation and representation of Adam Smith's ideas which took place over the 20th century, also generally misunderstood by the lay public thanks to an aggressive 60+ year marketing campaign by the Mont Pelerin Society and Atlas Network. But that's another rant.