Just thought I'd share with you an experience I had recently buying a Nikon VR lens from the Personal Classifieds section of this forum. I'm relatively new photography, so I didn't know that much about buying lenses beyond checking for dust and taking test shots at various aperture sizes and zoom distances. One of the things I didn't know how to test was the VR.
Anyway long story short after about a week I started to suspect that the VR was not working. Taking shots with and without the VR made absolutely no difference. After researching a bit more I realized that the VR was not functioning.. so I had to take it back to Nikon. My VR is now working, but after two weeks and $165. ( I didn't confront the seller because i) I only verified that the VR is not working several weeks after buying, and ii) I'm convinced that the seller wasn't out to cheat me.
So beware when you're buying a VR lens. This is how you test whether or not it's working:
1) Mount the lens, switch on your camera and the VR, and place the lens near your ear. Press down on the shutter release half-way. On many VR (not sure about IS) lenses you will hear (and even feel) a clunk and what sounds like water flowing. Release, and after a second you'll hear a clunk again and the sound of water flowing will stop. Try several times. The sound is quite unmistakable.
2) Set your camera to just one autofocus point, and adjust it so that the reticle (the small square representing the focus point) is at the center. Zoom the lens out to its maximum and focus on an object that appears to be about the size of the reticle in the viewfinder. Let go of the shutter-release. Observe how the camera shakes by seeing how the reticle moves about the object. Now press the shutter-release halfway. You should immediately see the image in the viewfinder stabilize. It's a very clear difference. If the camera is shaking just as badly after pressing the shutter-release halfway, the VR is not working. Repeat several times to ensure that the image stabilizes substantially each time you press the shutter release halfway. Again on my VR systems this will be accompanied by a slight "clunk" when the image stabilizes.
Hope this tip helps other people when they're buying a VR lens.