In case of Intermediary services
Under GST, You would be liable to pay GST@18% (CGST+SGST) on commission income. Under Income tax, you can show the commission income as income from other sources or Profits & Gains from business. Pay advance tax to avoid loss of interest.
As you have mentioned that seller will purchase from you. It seems that you will supply goods on your own account. In such a scenario you will not fall under the definition of intermediary.
For detailed discussion you may opt for phone consultation