Buying a used lens

I bought my 70-200mm F/2.8 IS and 100-400mm IS used and both came in excellent condition.  But you do have to be wary.  Once, at a birthday party, I saw woman lick her thumb and wipe her lens off with it, so all people don't treat their lenses with the same respect.  Granted, she was shooting snapsots with her Rebel in auto, mounted with an 18-55mm lens and she had no idea what it was.  This said, I would think that your highest risk would be from users with beginner equipment.  Those who pay dearly for the higher end equipment are probably more likely to take better care of it.  Although, I did recently purchase an 18-55mm IS, off CL, for my son.  He's now shooting my old 20D.  The seller allowed me to inspect the lens and try it on the camera.  It was in mint condition.  Good luck.