In my experience of buying and selling Glocks they normally hold a little under new value regardless of work done to them. Night sights and adjustable sights add no value unless you find the right buyer and wants to pay extra. The Zev trigger might bump up the price a little, but again normally aftermark parts and accessories don't add to a guns value since the buyer might not even want those features. Kind of like a AR15 with a scope, red dot, bipod, light, laser, hand grips, and so on. Just because the seller spent all the money on adding the features he wanted the buyer might not even care about those features and opt for a cheaper AR15 and do it exactly how he wants. You have to find the right buyers looking for that setup.
It sounds like you bought a good gun for 3 gun though, if that is your intended purpose you probably got a gun that will run with a lot of the other race guns and you don't have to do anything to it. That is sometimes the best route.