While I can't give you a value, I can give you some advice.
Start low -- ridiculously low during your negotiating. Don't try to beat them up on the price until they're in front of the bike though. A lot of guys get offended if you try to haggle over the phone without seeing the bike. You can always go up on your negotiation, but you can't go back down.
It'll also give you a feel for what they're willing to accept as far as price. Here's the real catch though, if you sense they are offended, you have to be able to play it off like you were kidding. I've gotten good at this over the years, and I've had to use it a couple of times.
If he has this listed in the $500 range, then I'd offer him $250 and go from there. If it's in the 600-700 range, then I'd offer him $350 as a jumping off point.
Good luck!