GST registration is mandatory for selling through an ecommerce portal. Normally GST registration is not mandatory if your turnover is less than 20 lakh. But that exception is not applicable to person selling through an ecommerce portal. It is because in selling online the customers will be from all over India. Any person dealing outside its state has to take GST registration mandatory even turnover limit does not increases. So GST applies indirectly due to selling outside the state.
Hence one cannot sell through ecommerce without taking GST registration