A lot of them are brought in by people who are also customers- I periodically have to go through my bookshelf and ask “Am I really going to be reading this again” until I have a big ol’ pile on the floor, then take the pile to the used book store. What they don’t buy goes to the Goodwill or Salvation Army. Then, of course, I start accumulating books again.
When I get a book jones, I usually hit the used book place first, then, if I don’t find what I need, I go to Borders. What I get at Borders often eventually wind up at the used book store.
Bookstores buy collections for the bulk of their books. This is as true for rare book stores that buy up million dollar collections stored in bank vaults as for used paperback stores that buy up a bunch of sf or romance that people want to not have to shlepp with them when they move.
Everything else is really secondary in terms of volume. I do get very annoyed when I go to the book sales that libraries hold and find that all the local dealers have descended on them in the first five minutes, hauling away boxes full of stuff before I have a chance to get my bearings. Still, a few boxes is nothing compared to a 1,000, 5,000, or 50,000 book collection.
I keep running into one of the owners of the local used book store at the Salvation Army and Value Thrift buying books there.
An auxilliary organization, :Friends of the Library" manages the annual book sales here with special privlidges for members first day. Then special pricing prevails with drops every day or two. Last day is a bag full for a few dollars. Good deal for all concerned. Biggest problem is finding a title you really want.
I have a good friend who runs a giant (100K+) volume bookstore in Wheaton, ILL, specializing in out of print and obscure Christian books.
They get called for private libraries upon the death of the owner, college and high school libraries trying to make room, etc.
One of his complaints was that Amazon lets you list ONLY 40k items in their used book section!
The internet has been a real revolution for them. Now their catalog is automatically accurate, instead of keeping a copy of the newsprint one they had mailed out, with many books crossed out so that customers could be informed the one copy had been sold, if they asked for it. With the ability to list tens of thousands of items for very little cost, it’s been worth it for them to go through the tons of old boxes they had, holding the “we’ll deal with these later” books from their decades of buying.
This is true probably everywhere. My complaint is that the Friends should bar dealers from their initial members-only day, but the ones around me don’t do so.
I understand it from their point of view: it gets a lot of books sold fast. But it’s frustrating to me.
Of course, there are library book sales and library book sales. The Friends of the Tompkins County Public Library in Ithaca, NY, own an entire building dedicated to housing their 250,000 book collection. They open it up for two weeks at a time twice a year and draw people from hundreds of miles away. Prices drop each day, but the good stuff always goes fast and first.