How to save selected data from listpicker using mysql /php?

<Border Background="White" CornerRadius="15" Margin="0,0,32,0" BorderThickness="1" BorderBrush="Gray">
         <toolkit:ListPicker x:Name="RetailerList" ItemsSource="{Binding Retailerslist}" Foreground="Black" BorderThickness="0"  Margin="30,-1,39,0" >
          <toolkit:ListPicker.ItemTemplate>
                 <DataTemplate>
                      <TextBlock Text="{Binding retailername}" FontSize="20" FontFamily="Arial"  Foreground="Black"  TextWrapping="Wrap" />
                  </DataTemplate>
          </toolkit:ListPicker.ItemTemplate>
     </toolkit:ListPicker>
 </Border>

protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
        {
            string selectedIndex = "";
            if (NavigationContext.QueryString.TryGetValue("scustomerID", out selectedIndex))
            {
                RetailerList.ItemsSource = App.ProductList.Producttype;
            }
        }

string url = "http://www.best.com/Best_Windows/insert.php";
            Uri uri = new Uri(url, UriKind.Absolute);


            StringBuilder postData = new StringBuilder();
            postData.AppendFormat("{0}={1}", "product_id", HttpUtility.UrlEncode(ProductDetails.productId));
            postData.AppendFormat("&{0}={1}", "customer_id", HttpUtility.UrlEncode(LoginPage.strcustomer_id_k));

            postData.AppendFormat("&{0}={1}", "retailer",HttpUtility.UrlEncode(this.RetailerList.SelectedItem.ToString()));
           
            WebClient client = default(WebClient);
            client = new WebClient();
            client.Headers[HttpRequestHeader.ContentType] = "application/x-www-form-urlencoded";
            client.Headers[HttpRequestHeader.ContentLength] = postData.Length.ToString();

            client.UploadStringCompleted += client_SaveUploadStringCompleted;
            client.UploadProgressChanged += client_SaveUploadProgressChanged;

            client.UploadStringAsync(uri, "POST", postData.ToString());

            prog = new ProgressIndicator();
            prog.IsVisible = true;
            prog.Text = "Connecting update to server....";
            SystemTray.SetProgressIndicator(this, prog);

public class BestinukRetailers
 {
      
   public string retailername { get; set; }
 }