No offence, but I suspect you don't quite get what a demo unit is. It is a HD800 S like any other, built to the same standards. If the dealer had left the box closed and sold it straight to you, that very headphone would have been brand new. However, your dealer opened it up to listen to it, or to let clients listen to it in his shop before deciding on their headphone buy. So in fact, you are buying a used pair - hence the discount. If it's carefully used for one day, this might be a hell of a deal. If it has been used, stretched, dropped, cord yanked,... for months, I suspect that would show on the headphones...
So it's not a purpose built unit off factory, like a pre-production model. The only reason for it being a demo unit, is the decision of your dealer to open up that specific box to start using those headphones for demo purposes...