Procedure for billing and filing under GST for sales of used car by business held as fixed asset

 The sale of used car by a registered person and applicable gst rate 18% or 12% as per 08/2018 GST Rate Notification.

If we opt for Marginal scheme, 

What is the procedure to invoice the difference between the sales consideration and Written Down Value of the car is positive and also which value to be shown in GSTR1 return. Either marginal value or sales consideration value.

Thanks,

Regards,

Senthilkumar