Your problem is that a power agent has sold the property in favor of your vendor even though the the GPA deed was not executed by a registered deed?
Well the power deed was executed in the year 1999 in favor of the power agent and subsequently the power agent sold the property to the current owner in the year 2005.
It can be seen from the above that it was not mandatory to register the power deed during that time.
It was made mandatory to register the power of atorney deed with respect to the transactions pertaining to immovable properties in the year 2011 by an order of supreme court.
Therefore it cannot be said that the seller of property was having a defective title in the year 2005, and thus the current owner is having a clear and marketable title to sell the property.
In my opinion, there may not be any problem to buy this property from the current owner provided all other aspects are checked and found correct by a local lawyer and has furnished the opinion with recommendation.