What does "Best Seller" mean in this context? Who says it's a best seller?

Thanks for asking. In fact, you can find all the evidence that it's a best seller on howisincorruptiblegoing.com

or you can look at this handy infographic: https://www.linkedin.com/posts/eries_incorruptible-new-york-...

"Instant New York Times Bestseller" [0]

[0] https://www.amazon.com/Incorruptible-Good-Companies-Great-St...