I mentioned this to the other guy but yeah they were destroying rare books. A rare book seller caught them by sneaking an Apple Air tag into one of his books that he shipped to them.
There's rare and then there's "rare." From what I've seen, the people involved have declined to say what the books involved were. About the only reason I can think of to hide that is because they know that people won't be as outraged when they find out what the "rare" books actually are. However, it was a bulk lot of something like 1,000 books, so it's a pretty safe bet that they weren't exactly valuable.
The 1973 manual for a toaster that I found in my grandfather's attic is certainly rare, but we're not losing anything if Amazon feeds it to their AI machine.