Yes agreed with Stoned.
When buying lenses whether new or used one: 1) Check the glass with a penlight shine into it and around the glass surfaces to look out for particles, bubbles, scratches, dust, fungus. 2) Use your camera to check all apertures settings (every single one!) and check the Depth of Field function too. 3) Check focusing whether can lock on near, far objects. 4) Make sure the zoom is smooth & at different zooming will give you correct perspective. 5) Check all the finishing, look out for mount damages, dents, cracks, etc. 6) Check filter threads 7) Shake the lens and feel if there's anything lose or screws dropped-out inside. 8) Last, haggles like mad to bring down the price.