I've seen a number of articles and letters in authors' publications bemoaning the existence of Amazon Marketplace. I think it's worth putting the story straight as it's by no means black and white.
Just to be clear what I'm talking about, if you got to the page for a book on Amazon, you will see over on the right something like this. It's an opportunity to buy new and used versions of the book from sellers other than Amazon. The mighty online bookseller doesn't do this out of the kindness of its heart - it takes a cut - but both seller and Amazon benefits from the purchase.
There are broadly three types of Amazon Marketplace sale. The first is new books from a catalogue seller. This is a company that's a virtual bookshop. They use an electronic catalogue and when you buy a book from them, they just order it direct from a distributor or publisher. From the author's viewpoint, this is no different from any other bookshop selling the book. The author will still get their royalty and all is well with the world.
Secondly there are one-off new sales. These might be unwanted presents, or a reviewer or author selling off unwanted copies. This is a relatively small group of books - not anything for authors to panic about. And anyway, often it's authors doing it. For instance if you look at the Amazon.co.uk listing for my book Upgrade Me and go into its Marketplace entry you will see the cheapest entry is from Creativity Unleashed, selling signed copies. That's me - and as an author I have no problem with it.
Finally there are the secondhand books. These, I admit, can be a worry for authors. If someone intended to buy a new copy from Amazon, with its accompanying royalty, and instead decides to to buy a secondhand copy, the author (and the publisher) loses out. This is true. However, quite a lot of people will prefer a new copy - and those who buy a secondhand copy may well not have stumped up the full price anyway. If someone buys a secondhand copy and they like it, it is likely to encourage buying the next book new when it comes out.
Best of all, secondhand copies are often of books that just aren't available new. Amazon now lists lots of out of print books where the only copies are secondhand ones. Better that a reader can buy one of these, rather than not at all.
I accept absolutely Marketplace will result in some lost sales. And it's sometimes a place of madness. So many people list books at 1p without spotting that this limits the amount the seller gets from the book to £1.58. Any large format book is liable to cost more than this to post - the seller will make a loss (or realize this and cancel the sale). However, on the whole I don't think Marketplace does as much damage to authors and publishers as many fear.