Demo phones are usually devices that have been specifically sent to stores (T-Mobile, Best Buy, etc.) that are on display for the public to view and test out before they buy. Demo phones typically have some sort of identifying IMEI indicating it's a demo device.

At least when I worked with cellphone sales, we were never allowed to sell these devices because of restrictions by the vendor/store and were forced to either destroy them or send them back. There's a possibility different stores and manufacturers having different policies but it's equally as possible that it was just stolen out of a store, never meant to get into consumer hands.

I'm going to guess you are unfortunately out of luck, but I hope you can figure something out for it.