Amazon surprised a lot of industry watchers last week when it announced that used product sales constituted 11% of total book, music and video orders. A feature that seemed useful mostly for clearing out Aunt Harriet's attic is now a revenue engine for the world's largest e-retailer? It seemed like a stretch.

Thanks to a 15% commission and low infrastructure costs, it's not. But as used book sales continue to pile up for the company, the more telling statistic may not be what percentage of Amazon's business comes from used books, but what percentage of used booksellers' business comes from Amazon.

"It's scary how fast it's growing," said Dave Strymish, co-owner of New England Mobile Book Fair, which runs the new and used cookbook outlet Jessica's Biscuit, among other services. He said he can attribute about 5% to 7% of his business to Amazon.

Until recently, used booksellers sold titles on their own Web sites or through collectives like Bibliofind. But they're finding Amazon, with its high traffic numbers and its clever way of pairing new and used titles, to be a far more effective Trojan horse. "It's absolutely first on our list," said Julie Marker, manager of Bloomington, Ind., textbook dealer Book Emporium, who said she sells 1,500 books per week on the service.

Perhaps the most dramatic example of how Amazon has given a cortisone injection to used dealers comes from Jeff Morrow, operator of the online book business It would be an understatement to say that Amazon has been good for Morrow's business. That's because his business didn't exist before Amazon came along.

"We kind of came across it by blind luck," said Morrow, who browses auctions and overruns, then buys the books and resells them via Amazon's blue boxes. Many of his titles are new or in mint condition, and are sold for 20% less than Amazon's price for new titles. (Amazon requires this as a minimum discount, presumably to deter homegrown entrepreneurs from making too large a profit.) At present, Morrow carries about 10,000 titles and hopes to quintuple that by year's end. He wouldn't disclose figures, but said he sells enough books to make a living.

In some ways, the trend is a poignant reversal of the affiliate model. When Amazon chairman Jeff Bezos started the program in the late 1990s, he hoped small sites would help Amazon. Now Amazon seems to be returning the favor.

Whether Amazon had this intention is a matter of debate. Certainly the "I-have-one-to sell" formulation of the blue box makes it seem like you're buying from another consumer instead of a professional. But dealers report that Amazon pitched the idea to them months before it launched the blue box last fall, which would indicate the company had hoped to become to used books what eBay is to concert tickets. At press time, Amazon had not returned a phone call seeking comment.

The growth of used bookselling on Amazon hasn't necessarily meant an overall increase in used sales; in some cases, of course, sales have dropped in other places. Still, mainstream quarters continue to furrow their brows. Industry chronicler Michael Cader last week expressed concern: "[A]s the Net facilitates easy and reliable used book sales, and POD keeps countless titles in print forever, isn't the market for new titles headed for inevitable diminishment unless we figure out how to increase the consumer base and/or provide greater value in new titles?"

And in a letter in the current Authors Guild newsletter, president Letty Cottin Pogrebin worried that the revival of the used book business will lead to lost royalties. She called on authors to lobby Bezos to remove the blue boxes and recommended that authors not link to Amazon. "It would be nice to inflict the kind of economic wounds on Amazon that Amazon's used book box inflicts on us," she wrote.

Used booksellers, for their part, have shown little remorse. "That's absurd," said Strymish, when told of the Authors Guild's argument. "Once something's been sold, someone owns it and can sell it. Is Ford supposed to collect money every time a used car is sold?"

At least some publishers have maintained a tolerant attitude. "Of course I want to sell every book fresh, clean and hard," said PublicAffairs head Peter Osnos. "But I also very much want Amazon to succeed, and I accept the fact that some of what they're going to do will make our lives a little less easy." As for Pogrebin's implication that Amazon is violating a trust between author and bookseller, Osnos said, "I read the most astonishing things from Steve Riggio [about booksellers not needing traditional publishers]. I would say that's a significantly more hostile point of view than Amazon selling secondhand books."