Gpa by sale agreement

First Owner "A" was alloted the plot by means of Hakku Patra, "A" gave unregistered GPA to "B".  Then "B" entered in to an unregistered sale agreement with "C", where in "C" has paid the full consideration.    Now "C", saying that he is a GPA representator of "A" has sold it to "D" by a REGISTERED SALE DEED.   Iam planning to purchase it from "D".        Can some expert please guide me of the consequences of this transaction. I can see that there is no GPA between "A" and "C".                                -Thanks in advance.