Check that this has been done...
http://support.apple.com/kb/HT201065
If the seller can't do this then likely he is missing a password, then the "someone else" you refer to might well be "the previous owner".
Needless to say you should ideally witness this being done...if it has already been wiped and a clean install done then you won't know if the seller had the necessary password(s) or not...