A couple of weeks ago I traded my 400mm 5.6 +cash for a 100-400mm with a guy on Craigslist. I did a lot of research before about tests to do on the lens and I did not see any problems with the lens. That weekend I wanted to try using the new lens to capture a bird with a pretty interesting call, so I flipped my 7D to video. After a few seconds the video stopped and the LCD displayed a message "Press STOP/START button to activate movie shooting." So I did that and it happened again any time I tried to shoot video with the 100-400mm.
I was with a friend that had a 100-400mm rented from LensRentals. I asked to try his and had no problem shooting video. When shooting stills, his image stabilization was much smoother than mine. When I tested the lens I bought, I thought that the IS was a bit wonky, but attributed it to the fact that it wouldn't be as good as my newer 70-200mm F4 IS. This made me think that there was something wrong with the IS. When I shot video with IS off there was no problem. As I tested it further, I found that when shooting stills, only with IS on, the camera would refocus (and reconfirm focus) every few seconds.
Long story short. I probably have a malfunctioning IS unit. I could have detected this if I had tried shooting video or live view (it happens in live view as well). So when you're buying used equipment I would add shooting in live view/video when testing a lens.
The seller refused to take the lens back. He claims that this is not really a problem and that I may have caused it in the day that I had it before I noticed it. I guess I understand this, but I still feel screwed because I traded him an absolutely perfect 400mm prime. He has offered to pay for half of the repair cost which helps lessen my frustration. Canon received the lens Friday and I'll let you know what they say about it. I hope that my story helps you when you are buying a used lens.