turbocharged - 2:51 pm on May 4, 2013 (gmt 0)
Popularity could include many different signals. For example, Amazon uses internal linking to promote similar products in their frequently bought together and items also purchased listings that appear towards the bottom of each product page. Also you can't discount offpage signals from affiliates selling certain items. Even though Amazon affiliates have unique URLs for the items they are promoting, rel=canonical appears on all of those affiliate pages to give Amazon maximum credit for Amazon's non-affiliate version.
I'm not sure what or if any information Amazon has shared with Google or if Google can pick up on all of the popularity information on its own. Since they are working together in some capcity (Internet Association), speculation among the internet community will continue. But it stands to reason that Google would want to serve its users results that end in transactions. With the domain name authority dial turned way up, combined with host crowding, the result is far fewer players in the game. Those competing with Amazon at this point, including those that manufacture items sold on Amazon, will have a tough road ahead IMO.