I've done CL transactions before, I'm not worried about getting robbed. I'm more worried that he may have OC'd the GPU and damaged it or something like that, and was wondering if the video seemed like good enough proof to you guys.
The Sapphire 5850 Extreme just came out 2 months ago (for $150 MSRP), it should have a solid warranty.
He's probably just an average gamer, I'd ask him to run a stress test for a few mins.
Edit: maybe a video which shows the serial, and some stress tests would be great.