Gabe Habash tries to calculate how many book sales it takes to become an Amazon bestseller, something Amazon refuses to reveal:
Like everyone else, [we] couldn’t get sales numbers from Amazon, but by studying the print bestseller list for a two-week period, we were able to determine that a title in Amazon’s top five averages 1,094 print copies sold across all channels, including other retailers, on a typical day. And because the general industry thinking is that Amazon accounts for about 30% of print sales, that means it likely takes around 300 copies per day to reach Amazon’s top five, depending on the day of the week and the time of year.
Or you could just hire a company to buy your way to the top (WSJ):
[Author Soren] Kaplan purchased about 2,500 [copies of his own book] through [a marketing firm named] ResultSource, paying about $22 a book, including shipping, for a total of about $55,000. Mr. Kaplan says he also paid ResultSource a fee in the range of $20,000 to $30,000. With 3,000 copies sold in its first week, [Kaplan’s book] hit No. 3 on the Journal’s hardcover business best-seller list. It hit No. 1 on BarnesandNoble.com on Aug. 7. By Nielsen BookScan’s count, about 1,000 print copies have been sold in the six months since.