Until a few years ago, anyone armed with a list of ISBNs could look up all the current and past sales and demand figures for those books, using Ingram Publishing’s 800 line, which has since been discontinued.
For example, my publisher Adagio Press looked up and recorded the top 100 books on Amazon.com at the time, then compared those figures with what the New York trade publishing companies were reporting in mainstream news and in other media.
The actual book sales were 10-100 times less than what the big publishers were reporting to the worldwide public.
For example, one of the Harry Potter books was reported by its publisher to have sold around 10 million copies then, but the Ingram sales figures stated approximately 785,000.
Not bad for JK Rowling, of course, but also not 10 million.