Buying used gear is always a balance between risk and potential reward. There is always a chance the lens will be defective in some way or another, but there is also the potential that you will get a perfect lens at a great price. When buying online, you are at the mercy of the seller, but when going out to buy used gear in person from local classifieds, there are several things you can do to decrease the chances of getting a lemon.
1. Stiff Focusing/Zoom Rings
Ages ago, I got burned by this one myself! Make sure those rings are smooth. When I fell prey to this easy-to-spot defect, I was thinking in my head that the lens just needed to be lubricated. After a conversation with my local lens repair guy, it turns out that stiff zoom or focusing rings are almost always signs that the lens has been dropped and damaged.
2. Dust or Mold Inside The Lens
Look very carefully through the lens when considering buying it. If there is anything “inside” the lens, consider that the lens might need to be completely taken apart to clean it out. Mold is especially bad. If you see any signs of even dust, though, it is probably worth running away with your money rather than opening that can of worms. Bring a flashlight to blast some light through the lens to make sure you can see anything that might be hard to see.
It is pretty easy for a nefarious seller to clean a lens extremely well to make it look “brand new,” but any slight scuffs or dents in the lens casing can’t be “cleaned” away and are a dead giveaway that the lens may have taken a beating in the past.
4. Poor Focusing
Take YOUR camera with you to see the lens. Mount it and shoot with it. Make sure to zoom into the shot on your LCD and make sure that it is consistently focusing where you have commanded it to. Some lenses can be notorious for back focusing or front focusing, which is something you probably want to avoid.
5. Included Warranty
I can’t believe how many times I see ads claiming that the lens has warranty left that you get to take over when you buy the lens. The vast majority of warranty guarantees only apply to the original purchaser of the lens. Don’t trust a promise of warranty until you have read the lens manufacturer's warranty agreement yourself.
6. Stolen Gear
Knowing when gear has been stolen can be a bit tricky, but there are a few things you can do to spot gear that may have been stolen. Obviously, if the seller has the original receipt, that is ideal, but often isn’t viable. Another great sign that the lens wasn’t stolen is if the seller has the original packaging for the lens, but again, that is often unlikely.
You can also try to get a feel for how “well” the seller knows the gear through common questions such as: “Why are you selling the lens” or “What did you use the lens for?” In most cases, a thief won’t be able to give you good answers or their answers will contain slightly inaccurate information. You could also try asking the seller if the lens would work well for something that you know it wouldn’t to see if they actually know what the lens was designed for. For example, if you are buying a very wide lens, ask if they think it will work well for taking headshots. A real photographer who is familiar with the gear will know it would be poor for that job, but a thief probably will readily agree to get you to buy the lens.
You Can Never Be Too Careful
Buying used gear is always a big risk. Make sure to be careful to avoid being scammed. The best thing to do is to always remain calm and not get too excited when getting ready to buy. Expect to be disappointed and walk away with your money. Never let yourself fall prey to the thought that the lens only has some sort of trivial, minor imperfection that can be cheaply fixed. When it comes to lenses, “cheaply fixed” is a very rare thing.