Here is a short checklist to buying 2nd hand lenses:
1) Look through the lens (dismounted) and at a light source so you can clearly see the front and rear elements (glass) with the maximum aperture. Remember to remove the front filter first.
2) Make sure there are no spots, smudges, cracks, etc. when you look thru the item (got spots might be fungus).
3) For normal twisting type, make sure twist motion is smooth when extending focal length.
4) Do a few test shots at different focal lengths and different f-stops to see if the picture is sharp. Pictures at different f-stops may reveal different defects, e.g. a speck of dust might show up at f/22 but not at f/2.8.
5) Make sure the aperture can reach maximum (i.e smallest f-number) and photos taken are sharp (make sure zoom in on your LCD and see).
6) Make sure when mounted, the lens is mounted firmly (normally there is a little lag, just a tiny bit but should not be able to twist the barrel, pls do not force twist)
Extra checks:
7) Prior to seeing the seller, ask them to send you some pics taken using the lens as well as physical pics of the lens. Ask him for the Lens code for Canon Lenses so you can check the age of the lens. The Nikon serial number should be able to roughly gauge the age.
8) Ask the seller if he can grant a 7 day personal guarantee just in case the lens breaks down within 7 days.
9) Make sure the seller agree to your paying price and not on the spot increase. If he increases on the spot, walk away.