If it's shown as 'used', you're almost certainly buying from Amazon Marketplace, rather than from Amazon itself. i.e. you're not buying the item from the big international bookseller but from a private trader who is using the Amazon website to advertise their goods. (It's similar to buying fixed price goods on eBay).
Just like eBay, Amazon Marketplace has policies in place regarding unsatisfactory items. Just like eBAy, most users of their site will stick to the policies but there's always a small risk that you'll encounter a dodgy trader.
Incidentally, if you're seeking secondhand books, there's a much better source (with a far greater range available):
http://www.abebooks.co.uk/