When shopping for a used lens it is important to know the value of the item and how that will compare to other options. A good place to start is at an online marketplace like eBay. Search for the exact lens you are looking for (in pristine condition), and then compare sold values to see what the price range is.
There are some other excellent places to shop for lenses, though. Some of these retailers specialize in only camera gear and are very careful to only sell quality equipment. Others are more general, but still often have great deals on cameras and lenses in top condition.
One of our favorite places to shop is KEH Camera and Photo. They are the premier retailer for buying and selling camera gear and have a stringent inspection process and detailed grading system. They also have a very generous return policy and warranty program.
Another good option is B&H Photo, who are a large retailer and also have a sizable used equipment department. They also use a rigorous inspection and testing process, and everything comes with a 90-day warranty.
Adorama is also a very reputable company with a very large retail website and a huge used equipment department. They are very careful to only sell high quality equipment and have a rigorous return policy and warranty program.
Finally, there is Amazon, who, unsurprisingly, has a wide selection of used lenses for sale. They have a “buy used” option as well as the more upscale Amazon Renewed program where you can choose to only purchase items that have been cleaned, tested, and refurbished.
When purchasing a used lens it is also a good idea to ask for proof of authenticity. Most of the major online retailers have a secure checkout system, but if you are buying from an individual it is very important to get all of their contact information, and then verify their identity through various websites. Also be sure to bring a laptop and a memory card reader when buying from an individual so that you can inspect the image quality on a computer rather than just an LCD screen.
It is also a good idea to take the time to read reviews for any lens you are considering. Not only can these help you find the best deal, but they can tell you whether or not the lens performs as expected in a variety of different situations. A few negative comments are not a big deal, but if there are many complaints about poor construction or performance it is probably best to avoid that lens.
Lastly, it is always best to buy in person if possible. This allows you to physically examine the lens, and in some cases even try it on your own camera to make sure it works. It also gives you a chance to meet the seller and, hopefully, get some additional insight into why they are selling the lens. They may be switching systems, or perhaps they have never used the lens at all.