Ever considered buying some second hand camera lens? twiching.com has written a post for the non expert.
Know what you’re after. Do a bit of research on the lens you want. Classifieds, eBay, etc will give you an idea of the price, and camera forums will give you an idea of common problems to check for with specific models. Both of those are probably more useful than most of the stuff below.
With that in mind, here’s some general steps for checking out a lens…