DataGridViewComboBoxColumn cell value returning null

I am trying to get a value from a DataGridViewComboBoxColumn cell that is named "Item".

When I run the program and make a selection from the combobox, string itemValue always returns "";

I am also getting an error on the line

cmbItems.ValueMember = productNames.ToString();

What am I doing wrong in trying to get the value?

My Code:

// set values to combobox column cells in datagridview
DataGridViewComboBoxColumn cmbItems = (DataGridViewComboBoxColumn)GridSellProducts.Columns["Item"];

cmbItems.DataSource = productNames;
cmbItems.DisplayMember = cmbItems.ValueMember;
cmbItems.ValueMember = productNames.ToString();
cmbItems.AutoComplete = true;

GridSellProducts.EditingControlShowing += new DataGridViewEditingControlShowingEventHandler(GridSellProducts_EditingControlShowing);

and

private void LastColumnComboSelectionChanged(object sender, EventArgs e)
{
      string itemValue = GridSellProducts.Rows[GridSellProducts.CurrentCell.RowIndex].Cells["Item"].FormattedValue.ToString();

      // get item price
      foreach (var item in itemListing)
      {
           if (item.name == itemValue)
           {
                    unitPrice = item.selling;
                    break;
           }
      }
}

private void GridSellProducts_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
{
     if (GridSellProducts.CurrentCell.ColumnIndex == 0 && e.Control is ComboBox)
     {
          ComboBox comboBox = e.Control as ComboBox;
          comboBox.SelectedIndexChanged += LastColumnComboSelectionChanged;
     }
}

Why can't I get the selected cell value?

productNames is a list of string item names.