using System;

using System.Linq;

using System.Collections.Generic;

using Microsoft.AspNetCore.WebUtilities;

public class Program

{

    public static void Main()

    {

        var ids = new long[] {1,2,3};

        var sellingToCategoryId = 200;

        var browsingToCategoryId = 300;

        var queryParams = ids.Select(id => $"sellingAttributeValueIds={id}");

        var url = $"http://example.com?sellingToCategoryId={sellingToCategoryId}&browsingToCategoryId={browsingToCategoryId}&{string.Join("&", queryParams)}";

    }

}