If you read in there, Si Dunn won't write the review unless he gives it 4 or 5 stars. He then sells the books he reviews back on Amazon. He doesn't do ebook reviews because you can't sell the ebook. So he gets paid about 65-75% of list. Looking at his reviews and twitter feed, it's a few hundred a month, and for a writer who quotes more than he reviews, probably not more than a few hours work.
So what if he makes a few bucks on it? The publishers don't pay him, they give him the same thing they give all the reviewers they request reviews from, a free book. He doesn't guarantee a good review in exchange, he guarantee he will either give a good review, a mediocre review, or no review. 4 stars on Amazon means the book is mediocre. If he doesn't like it, he doesn't review it.
That doesn't make his 5 star review illegitimate at all. If he thought it was a 4 star book, he would give it four stars, if he thought it was a 3 star book, he wouldn't give a rating. He gave it 5 stars because he thinks it is worth 5 stars.
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His note in full from your link:
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NOTE: I am not paid by publishers to review and tout their books. And I do not guarantee to publish a review about a particular book. If I do not like a book, I will not review it here. Writing a book requires significant time, effort, and expense, and I will not trash someone else’s writing. My goal here is to provide what I hope is an honest assessment and provide useful information to readers seeking particular types of books. Sometimes I offer a few critical comments, but mostly, I try to provide useful information that can help a reader decide to purchase a book–or not.
I receive sample copies from publishers, and I am under no obligation to publish reviews of any of those books. Indeed, I review only the books that I want to review, and I try to assess how well I think the books meet the goals set forth by their writers.
Publishers may mail printed books for possible review (no promises!) to: Si Dunn, Sagecreek Productions LLC, 1101 W. 34th St., #131, Austin, TX 78705-1907. As a general rule, I do not review e-books.
If you buy a book or e-book through links published in one of my reviews, I will make a few dimes (and not much more!) through my Amazon Affiliate account. Periodically, after printed books have been on my shelves for a while, I have to make room for new ones. So I sell some of the reviewed books on the used-book market. A few get donated to libraries. And a few are passed along to friends and relatives.
Book reviewing definitely is not a way to get rich quick–nor slowly. But reviewing is something I have done, part-time, for more than 35 years, mainly because I love books and enjoy helping authors reach new readers.
https://sagecreek.wordpress.com/about/
People can keep downvoting me, but it doesn't make that 5 star review any more legitimate.