using System;

using System.Collections.Generic;

using System.Linq;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        // sales data

        var sellerId = "157747190185796800";

        var listToReturn = new List<SalesDataRow>

        {

            new SalesDataRow { DateFromStr = "2020-08-25", Sales = 195000, Commission = 25350 },    

            new SalesDataRow { DateFromStr = "2020-08-26", Sales = 70500, Commission = 9165 },  

            new SalesDataRow { DateFromStr = "2020-08-27", Sales = 51000, Commission = 6630 },  

        };

        // shape the data into the structure we want for the JSON

        var resultToReturn = new 

        {

            sellerId,

            data = listToReturn.ToDictionary (

                item => item.DateFromStr,

                item => new 

                { 

                    sales = item.Sales, 

                    commission = item.Commission 

                }

            )

        };

        // serialize

        var json = JsonConvert.SerializeObject(resultToReturn, Formatting.Indented);

        Console.WriteLine(json);

    }

}

​

class SalesDataRow

{

    public string DateFromStr { get; set; }

    public int Sales { get; set; }

    public int Commission { get; set; }

}