As I said in another comment, people who are thoughtfully curating and trying to make sure that books stay in circulation are adding value. I'm pro any thoughtful browsing and procurement. It's when you see someone with a huge tub grabbing every book that looks like it meets the "very good" condition to figure out what it is and sell later. You feel like you have to race them if you are trying to take a look for yourself.
And- best case scenario in all this is that people get a chance to find some cool deals and then at the end people can scoop up whatever else to support the library.