Jim Behrle lays it out for you.
Weekly, the good folks at Harriet put up a post that links back to a list of poetry bestsellers. Where does this list come from? Is this a Publishers' Weekly bestseller list? Does the Poetry Foundation create a list? The word "bestseller" is a dicey supposition to begin with, across any genre. The New York Times bestseller list does ask many bookstores to report their own bestseller lists to contribute to the numbers. But a New York Times bestseller doesn't usually mean more customers bought #1 than #2. When a warehouse at a distributor replenishes another warehouse at a chain, that could count as bestseller numbers. And it's up to the individual reporting store to decide how to report to the New York Times. If a reporting store had events that particular week with Carl Hiaasen and Sloane Crosley, guess who will be at the top of their Bestseller List? Neilsen's Bookscan does not take into account sales at Walmart or Sam's Clubs or most independents. Many small independents either don't have the ability to report or don't wish to share sales info with potential competitors. Do Amazon sales # compute in people who buy new books used by some of the small stores that sell on Amazon? Probably not.
To look at a Bestseller List one assumes that all books were equally represented as possibilities for sales. But most of the Bestseller games are won in brick and mortar environments in Stock and Placement. Which is why the list Harriet highlights is filled with major publishers, the Bukowskis and Mary Olivers. These are the books generally on display, with more than one copy. Not the lone copy of a poetry book that is spined out in a small poetry section. If a publisher pays for a book to be well-displayed it will be in the major chains and some smaller indies. More.