Frame Or Paint Crack (Cannondale Super Six Carbon)
Since it's a few seconds with an allen key to pop the stem off and have a look at the inside of the head tube, I suggest doing that. Strongly suggest. If there's any damage to the fibres, or the cracks go deeper than the gel coat, don't buy the bike. If you don't know what that means, or the seller won't let you look, don't buy the bike. Exposed fibres or cracks on the inside of the frame might be fixable, but not by you (or you'd know enough that you wouldn't need to ask your question here, you'd be answering it instead). But they are probably not fixable.

The failure mode here is most likely that the head tube will break, probably when you're braking heavily. That's not a good crash to have, you can easily end up with severe dental trauma and that can never be fixed. Break an arm, grind half the skin off your face or hand, it hurts but those things grow back. Teeth don't. My partner has an expensive bridge where her front teeth used to be, and when we can spare $20,000 we will get them fixed better. Not fixed properly, because no-one can currently do that for any price, but something better than the "temporary bridge" that has been there for 10 years.

The trouble with buying something when you don't have a close relationship with the seller is that you have no idea whether it's really a good deal (if so, why doesn't the seller have any friends that will take the good deal?), or whether you're the gimp. Especially with carbon fibre and other materials that can fail in non-obvious ways, a common reason to sell is that the bike has failed in some way that's not going to be easy for the buyer to tell before they buy it.

The other obvious reason for a "good deal" is stolen property. Grab the serial number and search online for it, including the various stolen bike registries. You might be surprised at what comes up.