I've bought and sold on CL with no issues. As advised above, talk to the seller by phone first, if it smells wrong then pass. Meet in a public location in the daytime (hard to test a lens at night), bring your own camera. Test everything you can easily test. Sharpness is a hard one, since you may not want to bring a laptop and reviewing images on the LCD makes judging sharpness moot. But, shoot a blue sky or white wall to look check for decentering (vignetting should be symmetrical), test AF at close and far distances (slight misfocusing can be corrected by AFMA if your camera has it), test IS if the lens has it (slow shutter speed, shoot with IS on then off), check for smoothness of focus and zoom rings, if possible bring the correct size filter to make sure the threads are not distorted (common after a lens is dropped).
Personally, I pay no more than 70% of the current price of a new lens when buying on CL. That usually means I can re-sell the lens if I don't like it (which I've done many times, and overall I've made a profit on that).
If you see a good deal, move fast - a few times, I've found lenses for ~50% of new. Don't be afraid to counter offer, especially if you've seen the same ad listed for some time.