You have to file GST return to govt of India through its portal called gst.gov.in. You will be paying GST on the sales that you have made. Ebay has nothing to do with it. You can either file the returns on your own or take help of a Chartered Accountant.
In the gst return which will be GSTR 3B, you will be mentioning your total sales and total amount of CGST, SGST and IGST that you have to pay. You will also give statewise breakup of your sales and amount of tax that you need to pay. Other than that, you will mention any purcahse made from composite or unregistered dealer in GSTR 3B. You can also claim ITC on the items purchased and GST taxes levied on you by ebay India.
