Go for it dude, that looks great. I'd show up with 1000 cash, ride it, be friendly, and then offer 800 or 900. Maybe just take 900 to give you a negotiating edge if you're on the fence about it.
Be a little careful about comparing 105 and ultegra, that caad has 6800 ultegra, it is one generation old. The new r7000 105 is slightly more desirable than 6800 ultegra. Roadies are incredibly fickle about keeping up with the trends, and the new gen does have different ergonomics, aesthetics and shadow rear derailers with higher capacity. Also don't forget the most important part of buying a bike, that you like it for arbitrary reasons like the paint and stuff. Don't buy a bike that you don't like the look of just because it's high spec or a good deal or whatever, you won't enjoy riding it so much.
$1000 is a fair price and criticising a sellers bike is often the wrong way of haggling, but if he is a dick who pulls the 'i know what i got' and 'this is the second top of the line' you could tell him that it's not any longer.