I'm not a Nikon shooter, but the same principles apply for any DSLR system for the most part. If it reliably works on one and not on another identical camera, the problem is probably something wrong with the failing camera, though in theory, normal manufacturing variation from body to body could interact with a badly decentered lens in some way that lets it just barely work on one and not on the other.

In theory, an autofocus failure could indicate that:

• The contacts aren't making proper contact with the body (though t his will usually cause a complete failure).
• The AF sensor is misaligned just enough to cause longer lenses to fail on one body.
• The lens is decentered on one camera and not on the other because of a bent lens mount on the body.

or any combination of the above.

The first thing I would do is point the lens at a test chart using the working camera to see if it is significantly decentered. If it is, send it back, because that's not "excellent" condition by any stretch of the imagination.

The next thing you should check is whether the lens mounting plate on the failing camera is slightly bent from years of use. You can repeat the decentering test with the other body, and if you get very different results, you've found the problem. You might also measure the distance from contacts and other fixed points on the body to various marked spots on the flange using calipers, and if there's any measurable difference between the two cameras, you've found your problem, and a service center visit is probably required for that body.

If all those things fail, then it might be a misaligned AF sensor on the failing camera; I would not be surprised if that causes more problems on lenses with a longer focal length.

Either way, chances are you'll end up sending the body and the lens to Nikon together for repair unless you determine that the lens is badly decentered even on the working body, in which case you should just send the lens back.