These days the legal system has been tightened atleast i am sure in tamilnadu, you cannot sell without the owner's knowledge. but why would you give a poa to the person whom you do not trust. These days in TN, you have mention the property that you are going to buy/sell or for that matter, the reason you are giving a poa in the documentation in itself. The poa is only to the person and it does not have legal binding on a heirs claim. the poa can be revoked even before the death of the person or is deemed null at the demise of the person.