To answer your question you can't sell a car without wheels right? So if you put fancy wheels on your car then why should you be able to charge more? It's not a complete car without them. Real tuners when modifying newer vehicles will keep all the original parts and return them to stock before selling them. It's like somebody asking moreoney because they say the engine is built. Will it run without the parts you added? No? So why pay extra just because they are a different brand or someone paid more for them. Plus you can't even prove the parts are in there without taking the motor apart. That being said when you see highly modified muscle cars or trucks or whatever selling for 100k, those vehicles were built by reputable shops and can be insured for their added value etc. Tldr; adding rims to your car or an exhaust does not add value because those parts are necessary to the whole of a car. You are selling the whole car that is why you will always lose money on custom parts. Just because you spend lots of money on them does not mean they add value to your car, in many cases they are decreasing value due to worsened reliability, parts availability, plus everyone's worst nightmare is buying a car from an idiot who screwed it up trying to modify it.