Here is a short checklist to buying 2nd hand lenses. I've done some edits on 23 May 09:
1) Look through the lens (dismounted) and at a light source (or bring a torch light) 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, haze, fungus, etc. when you look thru the item (got spots might be fungus). Normally, there would be some dust in the lens. It is up to your comfort level how much dust in the lens you can accept.
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. Note that at a small aperture, e.g. f/22, a picture taken may also revel dust on your sensor (CCD, CMOS) is dust on t be the lens. To confirm if that is is dust on your sensor, use a different lens and shoot using a small aperture. If it is dust on your sensor, it will show up at the same position on the picture taken.
5) Make sure the aperture can reach maximum (i.e smallest f-number) and minimum (i.e. largest f-number) and photos taken are acceptably sharp (make sure zoom in on your LCD and see). Normally, the sharpest aperture for a lens is about 2-stops from the smallest f-number.
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).
7) For Nikon lens, make sure that the aperture is snappy. You can do this by flicking the lever at the lens mount area. When you push the lever, the aperture should go to the largest, and when released, the aperture should go to the smallest quickly. In some old lenses, there may be oil on the aperture blades, which may cause the aperture blades to get stuck or to slow down the speed of the aperture closing. This can cause over-exposed photos.
8) 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.
9) Ask the seller if he can grant a 7 day personal guarantee just in case the lens breaks down within 7 days.
10) Make sure the seller agree to your paying price and not on the spot increase. If he increases on the spot, walk away.