Offer him $450. That's what used Glocks go for around here. Sorry, but a used Glock is a used Glock, NIB or not. We're not talking P&R Smith and Wesson revolvers, Colts or generally anything with true collectible value. You kind of lose all of your resale value as soon as you drive it off the lot...
Believe it or not, I have a friend who was once told by a gunshop that they normally offer $350 for trade-in value on a Glock, any condition, vintage. Then they turnn around and sell it for $400-450, depending on condition.