And to echo krisdrum, how do you know it will fit you?

If it checks out, and it doesn't look like you'll have to do anything to it, it might be worth $500-600, but if you know that it needs a bit of work (especially if you need to change some parts to dial in the fit a little better) than you might start in the $350-450 range. Will the owner allow you to take it to a bike shop for inspection by a good bike mechanic? This mechanic might be able to give you an idea of its value. While replacing carbon can be expensive, it's easier to replace the fork than the seat stays if there's an issue. The problem with used carbon is that you will want to get it inspected by someone who can tell if it's structurally intact.

The benefit with having carbon in those places is to help reduce road vibration. They join together and form the place that holds the rear wheel in place. In the rear triangle of a bike frame, the seat stays connect the seat tube (the tube that connects the seat to the bottom bracket and crankset) to the chain stays.

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