My seller doesnt have the GPA doc (given by NRI)

Dear all Kindly request your advice on this case: Scenario : 1) Person A & his wife (NRIs at that time) buy a flat in Bangalore from the builder in 2005 ; Purchase Sale deed is signed by Person B (GPA holder of Person A & his wife) 2) In 2006, Person A & his wife sell this flat to Person C ; the sale deed is again signed by the same Person B (GPA holder of Person A & his wife) 3) Iam planning to buy this flat from Person C 4) EC taken at sub-registrar office shows clearly that Person A & his wife represented by their GPA was the first buyer and then the current owner is Person C Issue : 1) Person C does not have a copy of the GPA document of Person A & his wife 2) Person A is living outside the country ; he also is not able to find that GPA document and also does not remember if GPA was registered ; but he told he was unable to travel and he had given GPA from foreign country ; on the other hand, he is willing to support if any help required 3) Person A & his wife and Person B - all are NRIs Question : 1) If GPA document is not traceable OR If GPA copy is found, but not a registered GPA - Is there any other safe way to ensure the purchase which i will do now from Person C has no legal issues in the future ?