Amazon's book sales figures are among the best kept secrets in the publishing industry. Amazon updates their bestseller list hourly, but refuses to share data on what it takes to get a coveted spot at the top of the list.
Publishers Weekly recently did an experiment to try to figure out Amazon's book list by reverse engineering a book’s sales on Amazon and tracking its position from other book sellers.
Publishers Weekly concluded that a book in Amazon’s top five averages 1,050 print copies sold across all channels, including other retailers, on a typical day. The general publishing industry belief is that Amazon accounts for about 30% of print sales, which means it likely takes around 300 copies per day sold, straight from Amazon, to reach Amazon’s top five books.
Publishers Weekly reports:
We looked at Dark Horse’s The Legend of Zelda: Hyrule Historia, a 300-page collection of historical information, concept art, and chronology of the famous Nintendo video game series. We charted the book’s ranking on Amazon, and during the weeks ended February 24 and March 3, it sold 8,573 copies and 6,128 copies, respectively, at outlets that report to Nielsen BookScan. During that two-week span, Hyrule Historia never fell out of the top five on Amazon’s print ranking. For the week ended March 3, the book sold 6,128 copies at outlets that report to BookScan (which covers about 80% of print units).
To account for the units not reported to BookScan, we added 20% to its total of 6,128, which equals 7,353. Based on that total, the book sold, on average, 1,050 copies per day, which means that Amazon sold no more than that number of units on any given day. But because Hyrule Historia is available through other retailers like Barnes & Noble, Target, and even video game chain Gamestop—the book is also sold out on Walmart.com—the realistic estimate is that Amazon sold roughly 30% of the 1,050 copies, or 315 copies per day, on average.
Source: Publishers Weekly