I found this on-line. Google "How to buy used lenses". I think you'll get some good tips.
1. Check your source. There are many ways to buy used gear, from local camera stores, online retailers that sell used, eBay, Craigslist and photography message boards. Know who you’re buying from and how reputable they are, especially with eBay and message boards.2. Physical damage. It’s not uncommon for there to be minor wear from hands, sweat and the elements on a lens’s body, but dings are no good. Likewise, check for scratches in the glass on both ends.3. Shine some light through it. Hold a small flashlight at one end and look through the other end. Not all lenses are made the same and it’s possible for dust to get inside. While minor dust usually doesn’t affect picture quality, larger specks or clumps will and require an expensive cleaning process.4. Aperture blades. Change the aperture setting to make sure the blades inside the lens move smoothly and have no damage or oil on them. For most modern lenses you’ll need to mount the lens to a body to check them.5. Filter threads. The threads at the end of a lens to accept a filter can often become damaged from not mounting filters correctly, most commonly they get cross-threaded. This kind of damage you’ll want to avoid.6. Light damage. If the seller discloses there is light damage and it’s a cheap and easy fix, skip the deal. There is no such thing as a cheap and easy fix with lenses, they are almost always expensive.7. Contact points and mount rings. On most all modern lenses there are contact points on the back of the lens that pass the electronic information from the body to the lens for auto focus and aperture control. They are typically gold and if the previous owner didn’t keep the lens on a body or a rear lens cap on when not mounted to a body dirt, dust and grime can build up. While back there, take a look at the mounting ring and make sure it’s not damaged.8. Zoom. On lenses that zoom, make sure it does this smoothly. You don’t need to mount it to a body to check for this, but it’s a good idea if you can anyway. The lens should zoom smoothly, no matter if it’s a spin type or push / pull.9. Focus. Check the focus of the camera mounted to a camera, the auto focus should happen smoothly and crisply. It’s also a good idea to flick the switch on the barrel to manual focus and make sure that it too spins smoothly.10. Why? Ask the seller why they are selling the lens. There are plenty of legitimate reasons, such as switching from one platform to another, or they bought it and simply aren’t using it anymore, or they’ve upgraded. Sometimes the lack of reason for the sale can often indicate a hidden problem, so you’ll want to avoid it.
Good luck!
