Gst on used cars and commission

Hi everyone

I have a query. A dealer of second hand cars can pay tax only on the margin amount of the sale less purchase. For example he sold a car for 12,00,000 and he purchased for 11,50,000 then how to show in gst returns.
Whether it is
Gstr 1
50,000 in b2cs and 11,50,000 in exempt

Gstr 3b
50,000 in 3.a column and 11,50,000 in exempt outward supply

Or any other way because it has to match the books also.

Also, if the car has not been transferred in the name of the dealer but he still purchases from seller and sells to customers and then transfer the RC from party to party without his name in between, then would be the said transaction be termed as commission or normal purchase and sale?

Please help