Commission income

If your turnover is more than 20lakhs(10lakhs for specific states), then you are liable to get yourself registered and follow the GST law.

Here it is not supplying through e-commerce. They are just the intermediary who transfer the order and gets commision.

if you falls under defination of intermediary as per section 2(13), then its 20lac threshold limit applicable for registration

Since you are receiving the money, you should issue a tax invoice to amazon and ebay charging GST and discharge it to Govt. But 1st see whether turnover limit is crossing or not.