How To bind selected sequence order from listbox in ASP.NET

Question

<asp:ListBox ID="ddlTermsAndCond" CssClass="ddlTermsAndCond" runat="server" SelectionMode="Multiple" AutoPostBack="true" OnSelectedIndexChanged="ddlTermsAndCond_SelectedIndexChanged1"></asp:ListBox>

protected void btnTermsAdd_Click(object sender, EventArgs e)
{
string TermsNdCondID = string.Empty;
if (ddlTermsAndCond.SelectedIndex != -1)
{
lblMessage.Text = string.Empty;
foreach (ListItem item in ddlTermsAndCond.Items)
{
if (item.Selected)
{
TermsNdCondID += item.Value + ",";
// string testValue = item[0].Value;
}
}
TermsNdCondID = TermsNdCondID.TrimEnd(',');
DataTable dt = Entry.GetTermsDetailsMaster(TermsNdCondID);
grdTermsNdCondDtls.DataSource = dt;
grdTermsNdCondDtls.DataBind();
grdTermsNdCond.Columns[1].Visible = false;
// ddlTermsAndCond.ClearSelection();
// ddlTermsAndCond.Enabled = false;
}
else
{
lblMessage.Text = "Please Select Terms & Condition at least One";
lblMessage.Style.Add("color", "red");
}
}