The ABE=Advanced Book Exchange is a place for book dealers to list books for sale. They list books by title, author, paper/hc, price & condition. You contact the dealer & arrange for purchase. Sometimes the book has been sold, and the listing not yet removed from the board, but usually the book is available.
Often there are several copies of a particular book available, and you can pick between condition & location (like it's harder to pay by check to an overseas dealer). There is also something called Paypal that is starting to be used for accepting charge cards.

There are several other sites, but this is the place to start.
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The Advanced Book Exchange Home page