Fungus, haziness, how smooth the rings are, age of the lens, ever serviced and reason for that, bill/warranty if there is any...
This is what I can think of right now, if you google, you will get some more points.
The person who is selling it to me is saying that Zoom lens am trying to buying is giving some sound while focusing during night and it can be taken care of by giving to Service. Is it so?
Focus motor hissing noise (when trying to lock in low light or even in good light) is normal. I don't think servicing will fix that. But if the noise is unusual then may be servicing would fix it.