I think you will find that the Nitro cars are a little tougher to sell. E-Bay usually fetches the highest price but even a set up for yours (all the hop ups, motor, etc) goes for around $400 - $500 USD. Even then you will find that most of them don't end up selling.
I know there are a few guys in the club that would like to upgrade to a Serpent 710 but most already have a Nitro car and are more likely to be interested in the chasis which you can get for about $150 - $200 USD on E-Bay.... I think either way you'll have to drop the price or just hang onto the car.