GTA service vs sale of old or used trucks Vs GST applicability

My client is a GTA ie goods transport agency, having trucks of 100 plus, and under forward charge. we have sold certain old/ used trucks and the value of the trucks so sold are already got nullified after charging the depreciation in the books of accounts, which means book value of such trucks are zero. Whatever consideration got on sale of such trucks,is a profit in other words. I hope, this amounts to supply u/s.7, hence subject to GST which attracts @ 28 % , Am i right ?. Kindly advice.