They are buying up used books at rates that push up the cost of used books.
Used books are practically free to begin with. Libraries will often give away whole storage containers of books that they don't want anymore. Bookstores will sell books for pennies on the wholesale market.
Half-Price offers Books By The Yard to be used entirely for decorative purposes. $50 for 40-60 books, largely organized by color and binding style.
a book that might have otherwise become useful in the future
We have vastly overproduced the physical reading material relative to consumer reading habits. We have vastly overproduced certain genres and subject matter material. We have overproduced individual volumes. These books are not useful in any material sense. No more than the clothes we overproduce or the snacks we overproduce or the automobiles and electronics we overproduce.
We are swimming in trash. We produce mountains of it. We fill ravines with it. Stop treating hording as a virtue.