Yes as per section 24 CGST Act supply on behalf of E-commerce is , the registration is mandatory irrespective of turnover .
Even though its not sold on behalf of E-COMMERCE & its General sale ,the GST is applicable on profit only.
Note : Services provided on behalf E-commerce , no need to take compulsory registration if the threshold limit is not exceeding. (As per Noti.65/2017 CT)
**Moreover Generally E-commerce operator not deals with supplier who is not having GST No.
thats absolutely correct answer GST is not charged on profit. You just need to charge it from customers and remit it to govt. Additionally you would be eligible to avail ITC as well.
I meant that GST is not payable out of profit. In your example as well, you can calculate that profit of dealer remains 50/- (157.5-105-2.5) as desired by dealers. Thus, it will be wrong to say that dealer will have to afford 18% tax of profit after gst implementation. GST is an indirect tax and burden lies on final consumer only.