by selling anything off of it, the more complete it is the better price you'll get.
For that to make sense, he would have to get zero money for whatever he sells. That wouldn't be the case...often times you get more selling the "goodies" than you would if you left them on the car. Example: A pano top sold with the car = $0...the car is worth what the book value is. Sell the top separately, and you still get the value of the car plus $3500+ in your pocket. By not having the pano, the car as a complete unit only goes down by a couple hundred bucks, which doesn't come close to outweighing the $3500 you can get for it on ebay.
Similarly, nobody is going to subtract $400 or $500 from the value of a used car just b/c of a missing wind deflector. Unless they had already owned another R129, most people probably wouldn't even notice. Yet, I'd be willing to pay that...and there's the rub...it's extra money...
I always retail whatever upgrades or whatnot I do to a car separately when I sell it, otherwise you take almost a 100% loss on that stuff. When I sold my BMW, I put the stock rims back on and ebay'd the M-Parallels. I sold off the audio equipment separately. When I sold my Porsche, I got $30k for the car on trade-in, but I got $4k separately for the Ruf rims. If I'd left them on the car, I woulda got zilch for them, because the car was still worth what the car was worth, rims or not.
That's just the way the market works...nobody pays for options on a used car. To unlock the value, you sell that stuff separately.