One name to start this post with: Amazon. As you know, Amazon, just like many other online retailers works with a system of recommendations.
You bought 3 books on philosophy, 2 on marketing and 4 on some programming language? Amazon will regularly send you new titles in function of your buying history and show you some cool ads and recommendations when you visit the website.
The ads that consumers see on retailer websites are based on their buying history and what they do on the sites (even from third parties) and what pages they visit (behavioral targeting).
However, many retailers, including Amazon, also offer the possibility to buyers to add comments regarding the products they bought.