Pretty much this, although we used a seller account tool to set books up. There are many books not worth selling--mass market paperbacks and even hardcovers, cartoon books, many coffee table books, anything sold in great quantity. But some may surprise you -- I was into motorcycles and bought a bunch of books about back in the 90s. Turns out some are out of print and brought up to $75 or so. There were a few limited edition hardcovers I got hundreds of dollars for, but mostly, it was a steady trickle of $5-15 sales over a couple years.
But it does take time to set them up; process, package, and mail an order. Mail supplies, travel to and from the PO. If we couldn't list a book for $5 and still be on the front page of lowest price books, we didn't bother listing.