This has got to stop. The idea that no matter what you've changed on your car, it's worth less because DeMuro said so, is bullshit. Sorry Doug, I love you, but you're wrong here.
Yes folks, you modifiers you, you can breath because in some cases modifying your car can in fact add value! Will the value magically become the sum of the parts added plus the value?! No. No it won't. You see, many mods hurt the reliability of a car this the resale takes a hit. However properly done mods will still hold value, but parts depreciate as well. In fact they do so similarly to how cars do.
Example: Say you just bought a BRZ for $25,000. Then you call up Crawford and have them work their magic and you drop another $15,000 into it. Your car is not with $40,000. Your car is worth closer to $30,000 give or take a few grand, more likely take than give. So you've effectively added around $6,000 worth of value to your car by spending $15,000. And that amount will slowly depreciate at a similar rate as the car. So when your car is worth $8,000 in modified it may be worth $10-11,000 in its upgraded state, unless of course something is broken, worn excessively, etc.
However if you but a $1,000 exhaust, $500 tuner, and $1,500 wheels your car really hasn't gained much value at all. I If any. But for gods sake stop saying it will never add value.
Oh but one rule that hold true at least 90% of the time: Adding any EBay brand goodies to your car will never, ever, fucking ever, add value to your car. If you pull off an exception to this rule, please let me drive your car, you must be talented.
Photo cred: Motor Trend
Strange third person bio time? Fine. Shpuker is a third year engineering student at a liberal arts school in Michigan. Who the hell goes liberal arts for engineering? What an idiot. He owned a blobeye WRX for all of a year and succeeded in not breaking it, modifying it, fixing the modifications, and then not being able to afford keeping it. He also likes long walks on the beach and dogs. He thinks dogs are better than everything but boost, and he's right.
Disclaimer: This was all written on my phone, spelling/grammar mistakes are plausible and probable.
Another Disclaimer: Me and Doug really have a similar conclusion, just different paths coming to it.