I hate when people talk about the round count and that making it "just like new." I have seen some gp-100's that looked great, and had quite a few rounds and a few years on them go for $275-$300, and mechanically they were perfect.
If the gun has no marks on it, and has the hougue rubber grips on it, I might pay $375 - because like you said, it is used, no matter how he looks at it. And really, I hope that money isn't burning a hole in your pocket to give to this guy. Because if he wants more than that, you can EASILY find a used GP-100 in the $300-400 range. If it does come with some good accesories, then up to $400 wouldn't be a ripoff, anything too much above $400, and you are getting taken though.
If misc assecories are like cleaning stuff and a gun tote... those items really shouldn't add anything to the sale. Nice grips and nice holsters do make a differerence. But if you are talking hougue rubber grips for $20, and an uncle mikes holster for $15.... also shouldn't make much difference on base price of the gun.