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.