I made FiveStar [0], which was covered on TechCrunch [1] and it made ~$600 last month. Doesn't cost any money to run, so it just sits there. Not a fabulous success story, and traffic is decreasing, but $10 a day as passive income isn't awful.
i did a search for sennheiser and got an excellent breakdown of headphones. also i don't mind that a price range can have two different options if they're far enough apart. that's a nice touch.
I'd guess through Amazon affiliate links, which will pay out for every purchase that originated on the site. Very nice idea, and brilliant use of an affiliate scheme.
[0] http://fivestar.io [1] http://techcrunch.com/2014/06/03/fivestar-finds-the-best-pro...