public void StartBuying(string newItemId, int newItemMaximumPrice, int numberToBuy)
{
IStockItem stockItem = _warehouse.GetStockItemFor(newItemId);
var buyer = new Buyer(newItemId, newItemMaximumPrice, numberToBuy, stockItem);
stockItem.AddStockEventListener(buyer);
_portfolio.AddBuyer(buyer);
}