GST rate

if it is not practice of the seller(i.e., dealing in trucks) so the gst is applicable on "MARGIN PRICE" only , and not on the whole amount of transaction 

rate depends on cc , it may be 12% or 18% 

As per Notification 8/2018 CT (r)

Vehicle of Vat Regime Sold in GST Regime :
GST is applicable on Profit Margin (Sale-Puchase )

Vehicle Purchased & Sold in GST Regime :
If ITC claim , then GST is on transaction Value 
If ITC is not claim , then GST is on Profit Margin (Sale - Purchase)

GST Rate 12 % & 18% is applicable as per CC engine of vehicle mentioned in Notification.

More then 1500 CC , leanth exceeding 4000 MM, GST is 18%

Else ,.. 12%

So , calculate your Gst payable on sale of vehicle as per above said notification

thank you and congratulations