The New York Times printed an interesting piece about the economics of used books. Link. According to the article, used books probably help increase the sale of new books. Excerpt:
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When used books are substituted for new ones, the seller faces competition from the secondhand market, reducing the price it can set for new books. But there’s another effect: the presence of a market for used books makes consumers more willing to buy new books, because they can easily dispose of them later. . .
[T]he presence of lower-priced books on the Amazon Web site, [Amazon CEO Jeff] Bezos has noted, may lead customers to “visit our site more frequently, which in turn leads to higher sales of new books.” The data appear to support Mr. Bezos on this point.
Applying the authors’ estimate of the displaced sales effect to Amazon’s sales, it appears that only about 16 percent of the used book sales directly cannibalized new book sales, suggesting that Amazon’s used-book market added $63.2 million to its profits.
Furthermore, consumers greatly benefit from this market: the study’s authors estimate that consumers gain about $67.6 million. Adding in Amazon’s profits and subtracting out the $45.3 million of losses to authors and publishers leaves a net gain of $85.5 million.
All in all, it looks like the used book market creates a lot more value than it destroys.