I suspect big sellers must have dedicated account managers

No clue how it is elsewhere but in Amazon India, the largest seller is Amazon itself which sells under a different name. That's their model. They were under scrutiny by the Indian Government [1] [2] [3] last time I checked. Keeps registering subsidiaries under different names.

So you are basically competing with Amazon itself, which also acts as a seller in their own store.

[1]: https://www.reuters.com/article/business/amazoncoms-retail-p...

[2]: https://www.cnn.com/2020/01/13/tech/amazon-flipkart-india-an...

[3]: https://www.reuters.com/investigates/special-report/amazon-i...