Presumably the market for used books raises the price of new books.
If almost all new-book stores vanished, leaving us to buy new physical books online from middlemen or direct from the publishers, used bookstores would still be useful. Some would still be big enough or closely enough attuned to their customers to carry some new books. Publishers would get paid and editors and authors would be paid and we would all have market access to new and old books. People who don't have computers would still have somewhere to go. Good enough. I find that the local-culture, book-recommending, serendipitious benefits of bookstores are stronger in ones that carry used books already. Abebooks.com evidently keeps some specialist used bookstores afloat while letting them keep a storefront; that seems like a very good combination of internet one-market-to-rule-them-all and independent shops.
I hope that Apple and Amazon and Abebooks and Google don't become monopsonists and monopolists of books new and used, but that isn't really the same question as the death-of-new-bookstores. The chain stores seemed to be trying to become monopsonists, just not as effectively.
When Amazon and Macmillan trompled on their authors last week, several authors I like started selling some of their work online directly. Excellent; I am not willing to buy DRM-ed books, I am happy to pay a roughly-used-book price for electronic files if I have them forever, I am even happier if the authors get more money than for a new paper sale (which, remember, was higher partly because of an implicit possible used book sale).
But now I'm worried for used book stores, because even if I do have the right to transfer a digital copy -- beats me about the legal right, but they were sold, not licensed, so I think probably it's legal if I give up my access -- I'm not likely to be doing that out of a used bookstore, am I? Poof, there they go. I suppose this only happens if too many book sales are electronic. Query: what's happening to used CD stores?
Also, *this* is the point at which the 'marginal cost of transferring a digital file' is relevant to the price of a digital book. The original publisher/editor/author are going to charge something that keeps them alive and pays for commercially-reliable server space. Me, as a used reseller, I am presumably content with the pittance I now get for paper books from used bookstores. Doesn't that drive down the price of the 'new' digital version? Perhaps we assume people virtuously buy from the original authors for the same reason that they aren't warezing them left and right.
Maybe we get to buy books with a right to 'return' them to the publisher within a year, but not to sell them elsewhere. That's about the same as assuming that decent people won't pirate in a reasonable market. I think it might be true, and earnestly fail to disprove it myself, but it seems unstable for the authors.
Moreover, there's going to be some insane twilight-of-empire condition for books that were published on paper in the period of Effectively Eternal Copyright. Now I can imagine a world in which everything before 1928 is potentially freely available through Project Gutenberg and its later, sleeker imitators; and everything after 20?0 is 'natively digital' and gets sold at either the easy-legal-resale, high price, or the piracy-is-inevitable, fans-are-sponsors very high price... and everything between is sold and resold in increasingly specialized '20th Century Culture' shops, among vinyl furniture and records and collectible Pez dispensers.
This may be a more cheerful view than the one in which everything after 1928 gets so gummed up with DRM and viral copyright that we have to fork cultures and raise authors who are never 'legally contaminated' by exposure to owned memes.
I only meant to point you to two authors I like who have some work for sale: