GST is to be paid on transaction value. Sale of such old and used vehicle conform to definition of supply under Section 7 of CGST ACT. Here is an extract of FAQ dated 5.1.18.
"Q9. In transport industry, old vehicles, old tyres, scrap material etc, on which no Input Tax Credit (ITC) has been taken, are disposed of after completion of their useful life. As a truck owner disposing of these goods, am I required to pay GST considering that no ITC has been taken at the time of their initial purchases? Would levy of tax in such cases not amount to double taxation, as tax has already been paid at the time of initial purchases?
Ans. Under Section 7 of the CGST Act, 2017 supply includes all forms of supply of goods such as sale, transfer, barter, exchange, licence, rental, lease or disposal made or agreed to be made for a consideration by a person in the course or furtherance of business. Sale or disposal of old vehicles, old tyres and scrap material for a consideration would therefore attract GST regardless of whether ITC has been availed or not."
Compensation cess is not leviable w.e.f. 25.1.18 vide Notification No. 1/2018-C.C. (Rate), dated 25-1-2018 amending original Notification no.1/2017-Compensation Cess (Rate) dated 28-06-2017.
For rate of tax see Notification No. 8/2018-C.T. (Rate), dated 25-1-2018.