Gst is levied on supply of goods and services and selling of second hand car does not comes under the definition of supply thus no gst can be collected.
As per notification 8/2018 ( C T ) rate dt 25/1)2018 , GST applicable on Old and used vehicle selling , GST applicable on Margin ( Sales - Purchase ) value , So please refer notification for more details . Notification is available on CBIC web site / GST / Notification.