When people bring a rifle into the store to sell or trade we don't allow any additional just because it has a scope on it. The guy wanting to sell us the rifle may be unhappy with the way it shoots (maybe the scope or mount is at fault?). Even if it's a high end scope like a Leupold we just aren't going to allow anything over the cost of the rifle. So often the first question out of a potential buyers mouth is "how much without the scope?" Even if you're selling privately you are likely to get the same reaction. Sell the scope separately if you're selling, you will do better in most cases.

Really, the only exception might be collectible older rifles when fitted with a scope of the same era or original military sniper rifles still fitted with the correct scope. Collectible guns are a whole other ball game. Modern guns, don't expect to get the value of the scope to add anything to the value of the gun.