I wanted so badly to believe that this is a sweet old man with too much money and not enough sense and was just looking to give this lens a nice home
The broken English and little bits of photo jargon is what drew me in, but alas, I just can't see anyone throwing 10K out the window, even if they were a billionaire.
There are good deals out there, but I seriously doubt this is the one. I once met someone selling a 300mm f4L IS for $800 in like new condition on Craigslist. We met at a public place (her place of business; a bank). I walked through two feet of snow to get there, she felt bad, and sold me the lens for $700. That's a 50% discount! Lens still produces pretty good images: