public ShoppingCart Sell <T>(T requestedItem, int quantity) where T : BaseItem
{
var discount = DiscountHelper.GetDiscount(quantity);
return(new ShoppingCart
{
Item = requestedItem,
PurchaseQuantity = quantity,
PurchaseDiscount = discount,
PurchaseAmount = requestedItem.Price * DiscountHelper.ApplyDiscount(discount) * quantity
});
}