A has authorised B to sell the property on his behalf by executing a rgistered POA deed .
B, on the basis of the registered POA deed has transferred the property to C by executing a registered sale deed during the lifetime of A.
The transaction is absolutely valid and the buyer i.e., C becomes the absolute owner with clear and marketable title to the property by virtue of the registered sale deed on his name.
Therefore the seller i.e.,m C can very well execute a registered sale deed in favor of any prospective buyer in the capacity of the absolute owner of the property.
You may proceed with the purchase however to exercise abundant caution you may obtain a legal opinion from a local advocate before proceeding with the purchase.