Let me guess, they followed up by saying "but this XYZ never fails! and I have one in stock in your size". Cervelo bikes are fine, they probably don't have a higher rate of defects than any other big brand. Aside from the issues slowman mentioned, I've seen a lot of their R3 frames with the threaded bottom brackets with delaminating shells, some of the first generation BBRight R3's also had problematic downtube cable stops that would come unriveted.
As far as used bikes, I wouldn't buy one of any brand unless it was from The Pros Closet/store demo or someone I knew was selling it to me.
As far as used bikes, I wouldn't buy one of any brand unless it was from The Pros Closet/store demo or someone I knew was selling it to me.