Sylvia Nassar (who sits on the board of the Authors Guild and wrote the bestselling "A Beautiful Mind") has written a stirring defence of Amazon's practice of interlisting used and new books.
Well-organized resale markets have often helped, not hurt, producers of other durable goods. An efficient market for used books may allow publishers ultimately to charge more for new ones — and authors thereby to collect higher royalties — without necessarily sacrificing sales. Amazon's resale service has effectively split the market for new books in two: readers who buy and keep versus readers who buy and resell. They wind up paying different amounts for the same book, just as airline passengers pay different amounts for a seat on the same flight. Take, for example, Michael J. Fox's new memoir, "Lucky Man." The cost to the "business-class" customer who buys and keeps the book is $16. The cost to the "economy-class" customer is roughly $7, assuming the customer resells it for $12 and then pays Amazon's fee and commission. Splitting the market lowers the average cost of owning a book, creating more buyers.
Also, Tim O'Reilly has written a great note from a publisher's PoV:
Anyone who cares about books and authors should be applauding Amazon's expansion into the used book market, which is a real boon for consumers, and frankly, even for authors. As a publisher, I'm willing to take the chance that I'll lose a sale to a used book if that means that books that are otherwise unvailable can be easily found by someone who wants them.