I got the Carfax from the owner and the title is clear, no reported accidents or anything like that.  This is what the owner said to me in an email: 
The truck is in excellent working order, it has a clear and free title and I'm the second owner, I bought this truck in june 2011. The truck was garage kept, has no mechanical problems and it is in perfect condition. Description: Year:2007, Make: GMC, Model: Sierra 2500 HD, Transmission: Automatic, Exterior color: Blue, Interior color: Gray, Cold Air Conditioning, 83,265 miles, it has PS, PB's. ENGINE, DURAMAX 6.6L TURBO DIESEL V8, 4 Wheel drive, CD player, Driver airbag, Anti-lock brakes, Passenger airbag, Power locks, Power windows, Cruise control, Power seats . I uploaded the pics here: 
https://picasaweb.google.com/117748638052424307143/GMCSierra# the truck in the pictures is the one I'm selling you and that's the truck's condition right now. I have driven this truck almost daily for the last couple of months and it has been 100% dependable.
In another email he said he would ship the vehicle for free for the first 500 miles (he's in Virginia, I'm in Illinois), and that he would be in the hospital receiving chemotherapy for the next few weeks.  He said he wanted to be paid using Google Finance, and that I would only have to pay him if I decided I liked the vehicle.  
I'm still suspicious, even with all of this new information.  $7000 is cheap for any type of diesel around here, let alone an 07.