If this pre-owned N9 has not been re-housed and has original SIM card holder the product code (seven character alphanumeric) will be on label on underside. Trouble is that as Nokia's servers long since dead, compromises have to be made on flashing firmware bearing in mind that downgrading firmware is not allowed.

Whilst I posted this "Sticky" three years ago, it is not worth my while updating to 2021 as spare batteries and screws for disassembly much harder to source. Unfortunately flash memory chip is increasingly failing and battery discharges by itself if device left unused, requiring Harmattan flasher to re-charge it sufficiently to boot:
https://talk.maemo.org/showthread.php?t=100215