Well I've always told friends and family to NEVER buy a used computer on ebay, because you often end up "getting what you paid for" and get stuck with a computer that is riddled with problems.

I don't have any direct experience when it comes to 3D printers, but I would guess that generally speaking, a used one on ebay is probably going to be similar to a used computer on ebay: sometimes you might get a really awesome deal, but the rest of the time you probably would end up spending more in the long run to fix it or just end up buying a new one later.