1. Absolutely no dings, scraps, marks on the lens body and glass. Especially look closely at the front edges as this is where a drop will almost always make a mark.
2. Shine a light into the lens from both front and back elements to see if there is ANY inside dust, moisture, or the start of fungus (you usually only see fungus if the lens has been in very humid environment for an extended period of time and it often starts showing around the very edges of the glass first)
3. Try to find lenses that have been used in the studio only or came packaged with a new camera and not used at all. Having the original box and packing is a plus.
4. Check to see if the lens moves effortlessly (smooth as silk) when using the focusing ring AND zoom ring AND changing the aperture.
5. Does it come with a filter already on it and do they state they added the filter the minute they got the lens. What does the filter look like – clean, scratched, filmy, dinged etc.
6. Does the outside of the lens look new or dull – as in oil from hands, age, wear, being tossed in a camera bag, carelessly being hung around neck while galloping across fields etc. 🙂
7. Know who you are buying from and if they are reachable if there is a problem. Do they keep their equipment in a camera bag with separate partitions or just tossed in a bag or laying around attached to a camera body?
8. When buying from ebay, there is always the buyers protection, the pictures (make certain they are of the actual lens and not just stock photos), and your intuition (you know, that gut feeling that tells you this is a great transaction from a trustworthy seller – or not).
9. When you first receive a lens that you have not seen in person…go thru 1-6 above and shoot some images in a variety of lighting, focus, and zoom situations.
Generally, in a perfect world, you will buy used and get a lens that looks and operates like new and you might save a bundle.