I think you'll have to ask the seller if it is sim unlocked or not.
Nokia provides warranty for up to 24 months. So ask the seller whether s/he has the original receipt.
I bought a temporary nokia phone from Ebay, and obtained the receipt from the seller. When I took it to the Nokia Care point, they accepted it.
You can always say this was a gift, and so you do not have the receipt. They may still accept it...