I do a lot of looking online at places that sell used guns like Buds Gun shop, and also new ones and go from there.
Then you have those that are no longer made anymore which depending on the buyer may be worth more than the currently available models. Like MY C9 Comp that I just bought for $200. I considered that fair because I couldn't find any place online selling a C9 Comp and mom no longer offers the compensated version of the C9.
Realy, the "fair" price for any used item, be it a gun, a car, or anything else, can only be determined by the person wanting to buy the item. Also, the location the item is being sold will play a part on the value of it.