Search results for Kindle.
Search results for Nook.
There were also results for Sony and Cybook.
Apparently the way it works is that Amazon acts as the middleman between yourself and a third party, and you receive an Amazon.com gift card as payment. Check out the FAQ for more information.
I thought I'd pass it on as some people are bound to have old ebook readers sitting around, and older electronics are hard to sell. Given the flexibility of the Amazon.com gift cards, it's certainly possible to use it on things like digital music if one doesn't prefer to buy ebooks from Amazon.
edit - As noted in the comments at unclutter, markets like Craigslist are likely to yield higher amounts of money, should one sell directly to another buyer. Amazon definitely does not have the top dollar value for one's trade-ins. :)