Do aftermarket modifications increase trade-in value?

Usually not. Most dealers value cars closest to their original factory condition, and modifications like lift kits, custom wheels, loud exhausts or tuning can narrow the pool of buyers or even lower the offer. Some upgrades, like quality tires or factory accessories, may help a bit. If you have valuable aftermarket parts, consider reinstalling the stock parts and selling the upgrades separately, or look for a private buyer who appreciates them.