Gst valuation of Kirana store having multiple rate taxable supply

prashant rasal (52 Points)

12 October 2018  
Person having kirana store dont want to opt for composition scheme.
He purchased from registered dealer and supply to all unregistered persons.
How we can compute tax on such supply as he is not maintaining tax invoice for supply he made. But he has all records of purchased invoices. Please suggest.
Thanks