Right, that all makes sense but seems like it would be more of a grind than a simple JS API. You are effectively creating a marketplace.

Correct - see http://www.crosspixel.net for example:

"Cross Pixel's DMP is powered by our proprietary data relationships with more than 5,500 web sites and mobile apps where we identify and harvest the shopping and researching behaviors on over 650 million unique browsers. Our data partners are leading e-Commerce sites, search directories, comparison shopping engines, coupon sites and toolbars across North America and Latin America."

In general, the 'marketplace' is usually the DMP (Data Management Platform) where two parties can meet and share segments without data leakage (for example - Krux is a DMP used by a lot of Fortune 500 companies).

However the lines between DMP and Data Provider are blurring in recent years...

Great answer thanks! I hadn't heard the term DMP

BlueKai is one of the biggest; it's a data marketplace for cookie-tagged data. They were bought by Oracle a few years ago.

And Krux's website says they have been acquired by Salesforce.

http://www.krux.com/blog/general/salesforce-krux/

Big thanks for the info, that was insightful.