Not only can you find the K4, K3 (Kindle Keyboard), and Kindle Fire on sale, but someone is also selling every Kindle model including the original Kindle, the second gen Kindle (you can even get both the US and international models of the K2).
The sales are all refurbs and don't appear to originate from Amazon, but I am stumped by the idea that anyone else would have all this hardware in their warehouse. Seriously, the original Kindle was discontinued over 3 years ago. Where would you be able to get enough to have a sale on Ebay?
In any case, the prices on the early models are great. If you've ever wanted to start a Kindle museum, here is your chance.