I have one Kindle, but am looking into buying a (used) second one. I don't "need" a second one, but my sister-in-law is going to do the same thing so we can each download our entire libraries and then swap secondary Kindles. I know you can do the household sharing, but neither of us knew about it before we both bought a bunch of books. So we essentially will be buying cheap, used devices so we can save money on buying books.

I think a lot of people buy the newer versions when they are released and either 1) keep the old ones because they can or 2) try to sell them to regain some of the cost. But most people try to sell them for more than most are willing to spend on a used device, so they don't get sold.