The P.T. demo is gone for good on the PlayStation Store. Even if it remained in your library, uninstalled, you can never download it again. It wasn't a surprise that quick minded scalpers took to eBay with exorbitant prices for PlayStation 4 consoles with the demo already downloaded on them.
eBay isn't having any of it. They are issuing notices to sellers that their items can not be sold. Apparently, it runs against eBay's rules to sell PlayStation 4's with content or games downloaded to them. The same thing happened when people attempted to sell their iPhones with copies of Flappy Bird on them – eBay took down the listings.
eBay's policies don't exactly state such requirements on selling items with things downloaded onto them. In fact, they only suggest removal of downloaded content — they never say it is required. There's a bit of confusion within eBay out the policy that's being cited for item removal.
Either way, if you're looking to sell your PS4 with P.T. on it you might want to work up some code names.
"Selling PTS4!"
[Kotaku]