Your vendor has played fraud on you by suppressing the fact that this property was already under a sale agreement.
It may be said that after the death of the GPA the power deed automatically stands cancelled, but what about the registered sale agreement entered with the said buyer cum GPA agent?
Your vendor should have got it cancelled, but for wrongful gains he did not do so and sold the property to you fraudulently suppressing the existing facts of the transactions or encumbrances in the property.
It was your fault too that you did not obtain legal opinion from a lawyer just to avoid the legal fees.
Had you done it, you would have come to know about the existing encumbrance in the form of registered sale agreement.
Now no doubt you can initiate some criminal action against the vendor but it is badly barred by limitation for any such complaint.
You have to face the cases to be filed against you by the so called private financiers.