USNews Money column has an article titled “20 things you should never buy used”. Most of the items on the list are based on common sense but it also recommends not buying used digital and video cameras and used camera lenses for the following reasons:
… used digital and video cameras are likely to have been dropped and banged around. It may not be obvious, but once the damage kicks in, it’ll be expensive to repair. If you know what to look for in a digital camera, you can get a great new camera without breaking the bank.
An SLR camera lens is the most expensive part of a camera. It also directly affects the quality of your images. Any damage to the lens, however slight, will show up in your photos.
I think the advices above are misguided. It is true that there are risks of buying used items because of unknown damages that can lead to expensive repairs. For most used items, warranty, even if the item is still within the warranty period, may not be valid after the transfer of owners. However not all used cameras or lenses were abused by their previous owners. Many used gears were well taken care of. I have seen DSLR cameras and lenses that were used very little because the owners simply didn’t have time or they were purchased as backup cameras by professional photographers.
Secondly, cameras and lenses are built to withstand some levels of physical impacts. It is more so for the expensive professional grade gears. The chances of developing an immediate external damage are far greater than developing a slow internal damage.
The reasoning that any damage to the lens shows up in the photos is simply not true. The lens can have extensive wear on the outside but it will not affect optical quality. Even if there are minor scratches in the front elements the chances of seeing them in photos are almost zero.
Buying used typically means great savings when it comes to cameras and lenses. Certainly you need to exercise some caution and good judgement. If you are afraid of used cameras and lenses dying soon after the transaction, paying more for brand new gears is probably the right choice for you.