The biggest "risk" in buying used lenses is the well known issue of compatibility between Nikon/Canon bodies and Sigma lenses. If the lens you are interested in was manufactured more recently than the camera body you wish to use it with all should be well. If the lens is older than the body there may be a problem. The difficulty of course is that one cannot tell exactly which model of any advertised lens is being advertised. I have said before that Sigma make excellent lenses but the compatibility issues prevent me from even considering used Sigma lenses and the cost of new Sigma lenses, when compared to used Nikon equivalents is rarely sufficient for me to worry about. Not that I buy many lenses.