I just bought a used bike that otherwise looks clean and fine. I was setting it up on the trainer to dial in my fit, and I noticed that the cassette cogs looked funny when they were spinning.

It looks as though there is lateral motion when the wheel is turning or the cassette is freewheeling. At first I thought the bike wasn't hooked up to the trainer right, but it is, and it's apparent when there's no load on the bike and I'm just spinning the cranks backward. I don't think it's a cassette cog (or two) that's bent, as when you look from overhead, you can see the whole cassette moving.

What would this point to? I'm about to double check that the cassette is on right and the lockring is tight (though with the QR on and the wheel in place, it seems pretty solid, and I can't rock it by grabbing it with my fingers). Something with the freehub? Bent axle? Is the rear wheel toast?

The wheels on this bike were part of the selling point - I paid more than I'd like for a used bike, but I got everything I wanted - modern drivetrain (Ultegra 9sp), nice steel frame in my size (853, 64cm) and fast, light, tough wheels (2001 Ksyrium SSC) - so though I think the seller would take the bike back and refund everything, I've been waiting for this bike for a long time.

I uploaded a video, but this is with a low mpix camera and an unsteady hand - it's most obvious starting at about 12 seconds. http://www.youtube.com/watch?v=MfADB4-o4Is

Thanks in advance!