If the scope is a high-quality brand, it will add some to the value, but you will still take a beating on it. If it's a budget scope, it's essentially "free" when you sell the pistol.
The extra mags help the "saleability" but don't really affect the price.... same with the hard case.
The best way is to sell the bare pistol and sell the scope, mags, and case separately.
In my area, the bare pistol would bring $300 to $325 in a private sale if it's an honest 99%. The mags, about $10 each. The scope and case would bring about 1/2 new price.