I think that all that can be learn't from this is check stuff over when it arrives. In my opinion as soon as you change parts etc the bikes yours.
I was in a similar situation where i sold a bike and two months after getting positive feedback the buyer tried to claim £60 out of me because the rear brake started to leak, Its all about reasonable time. Had it been like that when i sold the box would have brake fluid on it and it would have been noticed straight away.
The damage near cannondale logo could have been done in transport or even when sat in you garage.
I've alot of experience working with carbon frames and seen alot of warrenty claims. Your frame doesn't LOOK like it would be any sort of problem the mark on the top tube is more than likely a scratch. the other damage to the paint and possibly the top layer of carbon. But thats all anyone can say from a picture on a forum, as said above take it to a Cannondale dealer and let them have a look.
I don't really think the seller has done anything wrong in this case, you could have helped him by inspecting the bike straight away which would give him the opportunity to resolve the issue fairly. So I think its an unfair representation of him as a seller.