Most books in distribution have sales coming from many avenues—author or publisher websites, the Itasca site, direct orders from bookstores and other retail outlets, Amazon (Advantage and/or Seller), and Ingram—a good representative sample but also not a complete list.
It is important to note that books moving to Ingram are not actual sales on the front-end. They are taken on consignment, and payment comes 90 to 120 days later for actual sales. So total sales are often a complicated and ever-changing puzzle.
Looking at inventory numbers in Stacks is a helpful place to start. I am also happy to go over specific numbers for any title. As in all other things related to the publishing, printing, and sales of books, there are exact numbers but to get to them, we have to take a multitude of sales avenues into account (ideally).