In case of used Motor vehicle - Notification 8/2018 central tax rate - 18% on diff between sale consideration and WDV as per IT Act ( if negative to be ignored).
Valuation in case of Capital goods governed by Section 18(6) of CGST Act. read as - In case of supply of capital goods or plant and machinery, on which input tax credit has been taken, the registered person shall pay an amount equal to the input tax credit taken on the said capital goods or plant and machinery reduced by such percentage points as may be prescribed or the tax on the transaction value of such capital goods or plant and machinery determined under section 15, whichever is higher.