That model is listed as $1819 brand new on Trek's website. So that means after tax and a couple of accessories out the door the guy probably spent $2100 and thinks that's what it's worth. The trouble is that most people who buy a nice bike like this think it retains most of it's value for several years. This is not the case. Also, "lightly used" doesn't really tell us anything about the bike. Any pictures? Anyway, if I was to run across that bike and it was actually "lightly used" and looked to be in close to mint condition I would consider $1300 reasonable. You have to remember, you're buying a late model bike from a private party and any warranty if out the door, you're on your own. Also, how clean it is and where the person stored it is going to tell you a lot about how they took care of it while it was in their hands.
What are they asking for it and can we see some pics?