{
  "2": {
    "sell_average": 239,
    "buy_average": 238,
    "overall_average": 240,
    "id": 2
  },
  "6": {
    "sell_average": 184434,
    "buy_average": 182151,
    "overall_average": 189000,
    "id": 6
  },
  "8": {
    "sell_average": 11201,
    "buy_average": 1723,
    "overall_average": 180,
    "id": 8
  }
}

public class ItemSummaryModel
{
    public string Id { get; set; }
    public ItemSummary ItemSummary { get; set; }
}

public class ItemSummary
{
    public int Sell_Average { get; set; }
    public int Buy_Average { get; set; }
    public int Overall_Average { get; set; }
    public int Id { get; set; }
}

List<ItemSummaryModel> models =
    JsonConvert.DeserializeObject<List<ItemSummaryModel>>(jsonSummary);

var dict = JsonConvert.DeserializeObject<Dictionary<int, ItemSummary>>(json);
var items = dict.Values.ToList();  //if you want a List<ItemSummary>;

public class ItemSummaryModel
{
    [JsonProperty(PropertyName = "2")]
    public ItemSummary Two { get; set; }
    [JsonProperty(PropertyName = "6")]
    public ItemSummary Six { get; set; }
    [JsonProperty(PropertyName = "8")]
    public ItemSummary Eight { get; set; }
}

var result = JsonConvert.DeserializeObject<ItemSummaryModel>(json);