It is mandatory for the sellers selling through ecommerce to take GST registration. However, for the time being, there has been a relaxation provided for small sellers who have turnover below 20 lakhs.
The registered sellers shall pay GST on the sale amount.
The e-commerce operator shall pay GST on his commission. And also on the sale by unregistered dealers.
Also, there are TDS TCS provisions for e-commerce operator. But these are not applicable right now.