The Mesa Public Library in Arizona has an amazing online booksale operation. In Fall 2019 we had the opportunity to tour it, and talk to staff and volunteers. Many thanks to Janice Dell, CVA, Mesa's Volunteer Coordinator who welcomed us and shared her materials, and to Jeffrey Stoffer, Mesa's THINKspot Librarian, who created the video.
This 10 minute video will give you an idea about how it works and how you might get started.
And a series of 3 parts of their training for the project: