Did you try logging into your amazon account that you bought it from?
Where did you purchase the device from? Also when you purchased it did you register it?
I am sorry to say that you won't be able to register it then. Without the serial number and without the person on ebay finding it for you then it is not possible to find it. Maybe he has an invoice where it bought it from first? If not I am sorry to say you can register it.
Did you ask him to check with the location that he bought it from? IF he bought it off of amazon then they will have it automatically if he registered it.
Good afternoon do you still need help with your fire stick serial number?