I'd probably steer clear of a carbon frame that old and I've bought two used Roubaix frames in the last few years so I'm not allergic to taking a chance with used carbon. However... carbon has improved pretty significantly in the last 10 years. The difference between my '07 roubaix and my '12 roubaix is striking.
So, $650 seems pretty expensive for used carbon, and you know there's no warranty support for resold frames (not that there is likely to be any on a frame that old anyway) but if you can get it for cheaper and it fits then why not?