Linq group by ID and then sum the ID itself (key)

I have two lists which are basically of the same class type like following:

   public class TransactionsTabResults
{
    public string ItemID { get; set; }
    public string Title { get; set; }

    public string ItemImage { get; set; }

    public float ItemPrice { get; set; }

    public int PageViews { get; set; }

    public float AveragePrice { get; set; }

    public int TotalSoldItems { get; set; }

    public float TotalRevenuePerItem { get; set; }

}

The list #1 is the TransactionsData and it contains 250 items whose only initialized properties are: ItemID, Title and ItemPrice;

The ItemID itself repeats several times inside the first List. Now what I'm trying to achieve in second list(transactionsList) is following:

 var _transactionsList = TransactionsData.GroupBy(x => x.ItemID).Select(pr => new TransactionsTabResults {
                            ItemID = pr.Key,
                            Title = pr.Select(x => x.Title).FirstOrDefault(),
  // here goes total sales of the item => sum of the groupby function by ItemID
// here goes the average selling price => sum above divided by price of the item 
                            TotalRevenuePerItem= pr.Sum(y=>y.ItemPrice)
                        });

As you can see I'm trying to group all the items inside the list by ItemID and then sum the the key to get the amount of sales per item (if you guys understand what I'm trying to achieve) - the amount of times that the ID repeats itself is the amount of sales that item has...

Can someone help me out to figure this out?

Edit: I need to get the key value (The parameter I'm grouping by the whole list) - value/amount of how many times ItemID repeats itself in #1 list...