99.9% of modifications decrease the resale value. That includes "upgrades" like stippling, triggers, cerakote, springs, etc.
Really? 99.9%? That's a lot! Not 87.8%?
A gun with night sights, an Apex trigger, extra mags, well done stippling and the original parts kept along with the original box and papers could very well bring you closer to the original price of the gun and possibly over it. With used guns you don't get close to the original price without extras and some quality additions to the piece. While some will turn away others, in the market for a used piece will look it over and pay more for the extras and the work if they want it and if the work is well done and the gun passes it's tests.
Getting close to the original price is a gain. A used gun that has been worked on in many cases exceeds the price of used guns without it in many cases. Obviously not all. Knowledge of firearms and what is and is not desirable and useful makes the difference.
There are gradations here and exceptions to set "rules". It's knowing the exceptions that makes the difference.
tipoc