Our local library-connected biannual book sale puts a large dumpster, about the size of an 18-wheel truck trailer, outside the warehouse where the sale is conducted. At the end of the sale it gets filled to the brim with waste inked cellulose, and sits there open in the weather until it's taken away for disposal.

Is that intended as an implicit threat - buy or we trash?

No, it's a demonstration of the lack of value of all that waste. On the last day of the sale (which decreases prices steadily over the several weeks it lasts) you can take a paper grocery store bag, fill it with books, and pay just $1. And yet enough books in the sale, enough to fill a large dumpster, don't even make that cut.