If you're in the US and considering a used Kindle, you might also check Goodwill's online auction site. Items are sold "as is" and they don't take returns, so there's some risk, but I've done well there. They always have some Kindles listed. Read the description carefully and make sure you get an estimate for the shipping and handling -- it can be significant.
http://www.shopgoodwill.com/listings...1&ending=Items