The CAAD3 was introduced in 1998, not 2007/8, so either the seller is way out with their dates, or it's a CAAD? and not a CAAD3.
Check out the vintage Cannondale brochures
HERE
ColinJ of this parish has an older CAAD, a CAAD5 IIRC. He rates it very highly I believe. You could PM him for his thoughts, but please bear in mind he's not too well right now so may not reply too quickly, (I don't know how active he is on here at the moment).
I ride the current CAAD10 and it's a fantastic bike, but the difference between a 3 and a 10 will be night and day, so a comparison with the two would be pointless.
If you're not used to a more modern bike then it should be fine. Price seems steep IMO if it is a CAAD3. Remember, it's still a 14 year old bike/frame, no matter what the condition.