A more general approach to building a marketplace would be to allow individuals to voluntarily register their interests and selected personal information with a web business, which then could then sell access (but not the information) to advertisers. This would be an alternative to Facebook and similar means for exploiting personal information while it would retain the individual's control of their privacy.

Facebook generally doesn't sell user information directly. Unless, you sign in to someones website/app or fill out a lead form.

You mean you want to build a marketplace of marketplace ideas? How are you going to get people to sell their information? :)

for that you can always create a marketplace of marketplace of marketplace ideas.

Cha ching ! => https://gph.is/2aHBxxB

basic attention token, Steem.io and a few others are working on this kind of stuff.