Valuation rule 32(5)

As per Rule 32(5) of CGST Rules, a Second Hand Car Dealer should pay GST on the margin earned (Sale Value-Purchase Value). So my question is when he is required to register under GST when he crosses 40 lacs Sales value or 40 lacs margin Value?
As per Section 22 read with section 2(6) of CGST Act, a person is required to take the registration if the value of taxable supplies exceeds Rs. 40lac.
Your Views are appreciated.