Depends on where you Buy them... A lot of the camera dealers sell used lenses... they usually test them and describe them very accurately... they also warrant the lenses to be as described... with money back guarantees of some sort... using a credit card or Paypal further protects the transaction... I have 30-40 lenses, and maybe 3 were purchased new...
Buying off friends, and Craig's list usually is risky because you don't get any of the protections in the paragraph above.
Your own ability to test and accurately analyze a lens is also paramount... I can't recall how many times I've seen folks say they bought a lenses and 2 years later they say, "I just turned on Vibration Reduction and it doesn't work"...or some other equally critical feature that probably NEVER worked and they didn't know it, and now the window to do a return has passed...