The lens is too young to have some sort of fungus.
Check whether there is any dirt in the optical pathway. This will pretty much not affect image quality at all (since the dirt is far, far, far away from the focal plane), but it is an indication of usage. Also, you may get a discount.
As Phrasikleia said, check the mechanics. I'd do some quick test at open and closed aperture and various focal lengths. Equivalently, you could also ask the seller to send you a few RAWs.
Personally, I bought all of my lenses used and have fared very well with it. Not once did I have problems with the lens afterwards.
Perhaps one more thing that may or may not be important: I have a sensitive sense of smell. I'd ask if the previous owner was a smoker or not. I've bought a flash from a smoker (and I didn't ask whether the owner was a smoker nor did he indicate it). I've spent one afternoon cleaning the flash three or four times with rubbing alcohol and I've washed the case in the washing machine. I'd ask. Obviously this does not affect optical performance, but I prefer to buy from non-smokers.
