Try and find comparable vehicles, or even trucks in general. Try to find say, two ford f-150s w/ 100k miles, one lifted/modified and the other stock and see what bids are doing online. Personally, I'd be more appealed to a truck (or even a car) that has upgrades-quality upgrades. $650 isn't going to do a whole heck of a lot, not even a new set of tires for sure. Do you have the $ to pay off your loan (1500)? If you'd rather not pay that, I'd put the $650 towards cutting that almost in half. How are you advertising? Maybe throw $100 into getting it on some websites and in magazines. If there is any damage (broken tail light, faded trim, etc.), take care of that and make it look new. Maybe get if buffed out so the clear coat is nice and shiny, try to get the most out of what you have to work with. You could even drop the price of the truck if you think you can swing the payoff. All about your situation and what you are comfortable with.