/// <summary>
/// Loads up the Brands Dialog Box
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void lnkBrand_LinkClicked(object sender, System.Windows.Forms.LinkLabelLinkClickedEventArgs e)
{
using (dlgBrand _dlg = new dlgBrand())
{
_dlg.ShowDialog();
}
}

It doesn't look as though your code is that which you'd expect to see in a databinding situation. 

In this case the each of the controls would be bound to a common BindingSource.

It looks as though you have a Listbox populated with some items and if you change the listbox item then a textbox is populated and a couple of properties.

Although its VB.NET code its mimicing what you are doing and works just fine.  Instead of selecting a selectedIndex of -1 I set the selectedItems property to nothing.   Ensure that multiselect property is set to false as you only want to be able to select a single item on the listbox.

Public Class Form2
    Private sBrandName As String

    Public Property BrandName() As String
        Get
            Return sBrandName
        End Get
        Set(ByVal value As String)
            sBrandName = value
        End Set
    End Property

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        ListBox1.SelectedItem = Nothing
        BrandName = Nothing
        Me.TextBox1.Text = ""

    End Sub

    Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
        If ListBox1.SelectedItem IsNot Nothing Then
            Me.TextBox1.Text = Me.ListBox1.SelectedItem.ToString
            BrandName = Me.ListBox1.SelectedItem.ToString
        End If

    End Sub

    Private Sub Form2_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Me.ListBox1.Items.Clear()
        Me.ListBox1.Items.Add("Fred")
        Me.ListBox1.Items.Add("John")
        Me.ListBox1.Items.Add("Bill")
        Me.ListBox1.Items.Add("Tony")

    End Sub
End Class

If your using databinding - you probably dont need to set the txtBrandName.text property in the selectedIndexChanged event - you can simply attach the datasource/datamember properties of this textbox control to the same data BindingSource as the listbox.

private void BindBrandList()
{
   DataTable _dtBrands = AAMP.DataObjects.ProductBrand.List();
   //Check and see if DataTable has rows and bind to ComboBox 
   if (_dtBrands.Rows.Count > 0) 
      {
         this.lbBrands.ValueMember = "id";
         this.lbBrands.DisplayMember = "brand_name";
         this.lbBrands.DataSource = _dtBrands;
      } 
      else 
      {
         this.lbBrands.Items.Add("No Brands Found");
         this.lbBrands.Enabled = false;

this.lbBrands.SelectedItems = null;

(threw an error saying the property or indexer is read only)

Tried:

[C# .NET] this.companyComboBox.BindingContext = new BindingContext(); // Bind the data this.companyComboBox.SelectedIndex = -