About 2/3rds of my lenses are second hand, always purchased form a camera shop.
Never buy a second hand lens that you can't have a good play and test with. Some people don't even realize that their lens id defective when they put it up to sell, and then you have all the scammers of course that mis-sell.
You need to thoroughly check the mount for wear and tear, that the rear element is pristine, there is no fungus or dust on the inside, the autofocus is smooth and silent, as is the VR. Checks the lens is well centered and doesn't have too severe front or back focus. Also check the filter thead is not damaged, common when people don't use a lens hood and drop the camera. Any of these things costs big money to put right.
The physical condition of the lens is not that important beyond showing whether the lens was well used or not, and the thing is some people use their lenses extensively but look after them and get them serviced. Other people rarely use a lens, drop it once which screws up the AF and they don't even realize. So it is a big like a car, something that is well maintained but cosmetically poor can be a better buy than a good exterior condition car that never had an oil change let alone regular service.
The front element is also less important but is worth checking it is not cracked, small chips and flaked off coatings has minimal affect, although can be used to talk down the price.
