I'm not sure if there's any difference between the 2014 and 2015 model but that would be worth verifying. I doubt disc vs rim changes the pricing that much, but that example from Pro's Closet seems wildly overpriced.

That said, $600 seems very cheap for a 6-7 year old carbon Ultegra name-brand road bike. Almost too good to be true. I'd approach with some healthy skepticism.