When you do a book search on Amazon using keywords, in the search result page you see 16 results. If you've bought one (or more) of these books, it still shows up on the search result page. It would be easy to eliminate that book that you already purchased (e.g. by showing it at the bottom of the page), and replacing it by another book that you haven't purchased yet. Sounds to me like an obvious, simple optimization trick. Maybe Amazon's statisticians use very complex models that change parameters of their search relevancy algorithms in real time based on thousands of variables, but fail to identify a simple change that should provide possibly a 1% lift to these particular users. At least they should A/B test my idea, if it is not too difficult to implement.
On a different note, they should refine their algorithms used to detect authors purchasing their own books, or authors paying friends to post good reviews for their books / bad reviews for competing books, or any type of fake reviews. Buying your own book is an easy way to get better ranking - just like clicking on your own ads on Google improves your position, as long as it is performed via 3rd parties to avoid detection. More on this later.