the
SELLER has all the words to say to explain to the buyer regarding the absence of the official receipt and other proof of legal ownership of the phone or any item for that matter.
the
BUYER in turn has all the options possible to accept or not the offer.
it's the
RIGHT of the BUYER to know the legal ownership of the item offered. since most of us are squeamish about asking the seller upfront the legal ownership of the item dealt, u can opt for a subtle way by asking for the official receipt when the item was first bought.