Best seller? What's that? How do we define the term best seller? I see it used constantly along with "award-winning author."
In the latter, if a group of eight people can vote your book is the best of the year for the Deadwood Writers Association, does that make you an award-winning author? Apparently some think so. To use the term loosely, cheapens the meaning and relevancy of the term.
My understanding is that in the early days of advertising and promoting, a book had to see at least 100,000 copies to wear that crown of achievement.
Today, publishers can use the term as they choose. If most of their books sell fewer than 2,000 copies and one of their books soars to 3,000 copies, they call it a best seller. For them, it probably is.
Someone, quoting Dan Poynter (often called the self-publishing guru), says that if a book sells 35,000 copies it's a best seller. I'm not sure where he came up with that figure.
As a general rule, being on the best-seller list of the New York Times, USA Today, or having a high rank on the Amazon.com list seems to be enough to call your book a best seller without counting the copies sold.