>I imagine they are only buying one copy of each book

The previously struggling second hand bookseller in my town has upgraded their car from a 15 year old hatchback Renault to a brand new Range Rover. Some Canadian company has been buying any book he can provide them for the last year. Their quotes aren't by number of books, or even weight, but by volume. As in, they pay him by the shipping container, and he sends several of those a month.

I think it's reasonable to assume the books in this supply chain which aren't destroyed in digitisation are just pulped and sold to Procter & Gamble for toilet paper manufacturing. I can't see any other fate for Anthropic's second and third copies of The twelfth edition of Vera Lynn's 1980's memoir "We'll Meet Again".