Here's what my uncle told me about tuning cars: when you trade a modified car to a dealership it sends a big flag to the next purchaser that the car was likely hooned by a 24 year old flat brim wearing, monster energy drinking sterotype. Whether or not it's true is not the relevant part, but the perception is. So the dealership has one of two options; lower the selling price to try and attract another 24 year old flat brim wearing, monster energy drinking guy with a little less money OR they go buy all the OEM parts and throw those sick mods in the trash, so they can get more money for the vehicle. Sure, those parts are valuable to guys like us, be to a dealership they actually hurt the value of the car.
Which is why we're telling you, part out your car and you'll get more money.
Extreme example; which one of these do you think a dealership could sell easier?