Hi,
As per the business model explained by you, you are doing sale purchase of goods through online platform. As such you will be considered as normal trader as per the GST Laws.
Answers are as below:
1) You can take registration after your turnover crosses the basic exemption limit of Rs. 20 Lakhs. No Compulsions for this. However, no benefit of input tax credit be allowed till you get yourself registered. However, since you are importing goods from China, GST Registration is compulsory for you.
2) You will have to follow guidelines related to Foreign Trade Policy and Customs Rules and Regulations for import of goods into India. Bill of Entry, AWB/ BL etc. to be submitted to Customs for clearance of goods after payment of applicable customs duty on the same.
3) You will be liable for payment for customs duty since the invoices by the Chinese seller are in your name. Bill of Entry shall be generated for each consignment separately, hence you have to pay customs on each consignment separately. No concept of separate monthly payments. However, you can make advance payments, provided the customs port are same for each consignment.
4) If you sell the goods outside India, you will be a deemed exporter. But I believe in that case, goods will not land in Indian territory since the seller is also outside India. Hence no custom clearance in that case. There is a concept of "MERCHANT TRADER" as per FEMA in this case. You will have to raise invoice in foreign currency.
Regards,