That is a nice bike, but I wouldn't pay $600 for it. I'd be thrilled to pay $500 for it if it fit. Keep in mind that a bike that old is probably only going to have 9 gears in the rear, whereas a new one would have 11 and even a new entry level bike would have 10 (as would a similar bike of the higher groups made in the last 5-6 years). That's not really a huge deal but it's a significant change in technology since that bike was manufactured. Also, while that bike has 105 shifters and an Ultegra rear derailleur, as you can see from the pictures, the company substituted a cheaper crankset and brakes. The crankset is the heaviest part of the group and where most weight savings comes from, and it's also the most expensive part of the groupset to buy when building a bike, so that is common. That's also not a huge deal but it's something you should consider when looking at other brands that will use the more expensive Shimano crankset.
Do you want a triple? I love them but many roadies are obsessed with weight and prefer the lighter bikes with a compact front. I prefer the triple because I like always having the right gear; every bike in my stable has a triple, even my carbon road bike.
Also consider that carbon bicycles are delicate and the weight savings is minimal. An aluminum or steel bike will survive a crash better. The good news is that you're getting a lot of used bike for $5-600 so maybe that's not a huge concern.
I say at least go look at it and see how firm he is on the price. Inspect the frame carefully, though, because while carbon is extremely strong, once it's compromised, all bets are off.