I go to plenty of used book sales at libraries and these guys are annoying as hell. They scan and hoard a lot of the books irrespective of the topic, and later spend time looking at each one to check its resale prospects. What they don't want, they dump back, not necessarily where they were supposed to be, and head out.

My library has even started posting flyers saying hoarding is disallowed. Libraries have also started not bringing out all the books at once, so that once these guys have done their "business", there's more quality books left for the rest. As for me, I think they should just ban the use of these devices at these sales.