Service tax

GTA service has undergone some recent ammendments vide Notification No.30/2012, Refer the Article -Service Tax a new Approach under the Articles Section , It is very clear and simple.

If you are paying service tax on GTA services under Reverse Charge then you cannot utilize CENVAT Credit for such payment. 

You have to pay service tax on GTA services in Cash and only after paying you can use such amount as CENVAT credit which in your case would not be possible since you are in trading.