The garage sale claim may be plausible... I know some dude who got a Canon 50/1 from an deceased estate sale for $350...
Oh and then a D5 for parts? For $150? Who is he kidding? D700 for parts can go for more than that. Not even one year, someone already managed to destroy a D5... yeah right....
And then replacing the buffer boards... I'm sure the firmware of the D810 would not allow that, it may even not recognise the chips on the board, let alone being able to communicate with it!
The person in question also showed absolutely 0 tangible evidence, it's all just words!
I have a unicorn in my d810.