I'll bite.

How do you know?!

 

I can tell you about the cars I've sold and what I was offered before and after a detail, or one can easily cite the numerous examples on Autopia.org regarding the same exact issue, keeping in mind there's an entire community dedicated to this topic. Sometimes I read this forum and it's like visiting another planet. Sure, detailed cars are less appealing to perspective buyers. It's like arguing that foul-smelling food is more appealing to diners. If we repeat it often enough, who knows, it might even come true.

 

I understand I'm in the minority with my opinions of a freshly detailed car, but that doesn't mean my opinion is invalid. Given 2 identical used cars, I'd buy the one that was vacuumed and wiped down by the owner over the one that's been detailed, simply because I don't like the oily crap that's all over the interior of a freshly detailed car. If the engine bay has been detailed, it looks like they're hiding something to me. But like I said, that's my opinion.