I saw this and took a look too, but I got offered little over 9k for my 53 plate, GT pack with Rays on their online valuation (see link in sig).
With the market as it is though, it seems to be very tough to sell via a private ad, at least here in NI anyway.
I suppose at least with these sort of schemes you can expect the money to be in your account within 24 hours, no messing about/risk etc. That said, you take a substantial hit on the value of the car.
However, I'm surprised that it was more than the trade in value at Nissan! Did they give what looked like a good deal on the new car and then try to take the **** with the trade in or some such?