I can only help with part of your question. “Stripped” books, books without the cover, are reported to the publisher as unsold. The bookstores tear off and return the covers only to the publisher, and receive sales credit for them. Sending just the covers back saves on shipping, for one thing. Thus, the author makes no royalties off those books. So if you BUY a stripped book, someone else IS making money off what is essentially a book they got for free. The rest of the book is supposed to be destroyed, and some places do bin them for recycling. From what I understand, it’s a given in the industry that these stripped books are frequently taken home by employees.
I have no idea how long they sit on the shelf before they’re stripped, but I know when my sister worked at Waldenbooks they did this about once a month. I used to get loads of stripped paperbacks from her, then pass them along to friends. As long as no one was selling them, I believe we were within the letter of the law (we were stretching the spirit of it quite a bit, though.)