Unroll.me is a popular inbox tool for unsubscribing to email newsletters. They monetize by selling your data through Slice Media (and don't really talk about this on their marketing site). In Mike Isaac's profile on Travis Kalanick of Uber, it was revealed Uber engineer/growth folks bought this anonymized receipt data.
Original article: https://mobile.nytimes.com/2017/04/23/technology/travis-kalanick-pushes-uber-and-himself-to-the-precipice.html