your contention that the vendor and vendee cannot be the same appears to be misconceived
the vendor is acting through a GPA holder and the vendee is the GPA holder himself
this does not make the vendor and vendee the same person
the agreement is essentially between the vendor [acting through his POA ] and the vendee [who is the POA holder himself]
this is not barred under the law
here though on the face of it, it would appear that both parties to the agreement are the same person, but it is not so
to repeat the vendor is acting through his POA holder and the vendee is the POA holder himself. so these are two different people
if consideration is passed on from the vendee to the POA holder of the vendor and the POA holder has not accounted for that money so received to his grantor, then the grantor of the POA has to sue the POA holder and also revoke his POA
But that may not render the sale effected through the agency of the POA holder as null and void, unless the owner proves that the sale was fraudulent with intent to cheat and deceive the owner
also the landowner, if he has not received the consideration from his POA, then he can also ask for cancellation of the registered deed and restoration of possession [if possession is also passed on]