I just sold my K3 on fleaBay - glad that a few people there were willing to take a chance on me!
In all seriousness, it's like any other used item - cheaper price, but a higher risk in case things go wrong and no warranty. In my case, as a seller, I sold it only because I prefer the smaller size of the K4, as that one can fit in my back pocket. So my K3 worked fine, I just wanted it off my hands since I figured it was going to just sit on my shelf.