While I'm not speaking to the validity of this phone, you can't equate this to a normal Nexus 6p. This is apparently a special edition of the device, probably only available to attendees of the event mentioned in the letter shown in the pictures. A person just looking for a Nexus 6p for daily use isn't the target buyer of this ad, it's someone who wants a rare or exclusive version of it. Limited and special editions will always command more money that those from the standard run.
But like I said, I'm not speaking to the validity of the seller's claims, or even that it isn't stolen like some here have said. However, it's not unprecedented either. I've seen special editions of devices given out at events that later on find their way to eBay or swappa. The seller can recoup the costs of going to the event. Same as when you see the special Employee Edition Droids being sold. Those editions aren't available to regular consumers, so if you want one, they're going to cost you more than the ones that are available to everyone.