This seems obviously incorrect to me, since Napster allowed illegal distribution of pirated music, while Amazon Marketplace simply allows one to sell a legally purchased book or CD to someone else -- a perfectly legal action. Follow this argument far enough, and you'll end up banning libraries.
On the other hand, this is an example of easy searchability perhaps taking a process that worked fine in the analog world and making it a little too efficient -- just as putting drivers' license and other publicly accessible state records online makes people more nervous than knowing they're available for examination in the county courthouse.
Amazon, interestingly enough, uses a defense that's frequently heard in discussions of online music: "The lower prices of used books allow people to experiment with authors and genres in ways they might not have otherwise."
So is this something that authors should be concerned about? And if so, what could be done about it?
I should note that I've unloaded more than a dozen items, mostly CDs I had purchased to obtain digital versions of LPs I already owned, through Amazon Marketplace recently. It's better than eBay in that you don't pay any fees if the item doesn't sell and it stays listed for up to 60 days. However, they really clobber you with fees: they take 15% of the selling price, and then only give you $1.84 out of the $3.00 shipping charge they got from the buyer. I haven't the foggiest idea how someone makes money selling a paperback for $0.15 this way.