1)builder has to sign sale deed in favour of purchaser .
2) A cannot sell flat to you . mere signing of builder as witness wont suffice .
3) builder can give power of attorney only in favour of relative . please note that Any registered property owner can execute a GPA in favour of their spouse, son, daughter, brother, sister or any other relative to manage his property or empowering him to execute any further deed of transfer including conveyance, sale and gift deed".
4) flat cannot appear in name of GPA holder without sale deed . inorder for GPA to be lawful owner of flat sale deed is necessary .
5) occupation certificate is issued by muncipal corporation . if construction of building has been approved by panchayat check whether building has been constructed as per sanctioned plans .completion certificate has been issued by panchayat or not
6) contact a local lawyer