Threshold limit for obtaining GST registration?

I am a retired person and want to supplement income by selling 3rd party goods on e commerce platform be it Amazon or Flipkart.
But they insist on GST registration for enlisting me as a seller.

I understand that till 50 lacs annual turnover over there's no need to register under GST.

1) Please guide how to deal with this situation.
2) Can one register under GST with out a consultant?
3) Do I have to file regular return even if turn over is much below threshold limit?
4) Also looking for Mumbai based consultant in this matter. TIA