As per law, your grandfather's self acquired property became intestate upon his death. The intestate property will devolve upon all his legal heirs, i.e., on his two sons and 5 daughters equally. The legal heirs of the deceased heirs are entitled to inherit the respective share of the deceased heir. Therefore for disposal of the property, the signatures of all the living legal heirs of your grandfather and from the legal heirs of the deceased heirs of your grandfather are to be obtained which is mandatory in law.
If the buyer is insisting on the khatha in the name of the vendors, you may have to apply for khatha from BBMP.