string strFormValues = Encoding.ASCII.GetString(this.Request.BinaryRead(this.Request.ContentLength));
string requestUriString = "https://www.sandbox.paypal.com/cgi-bin

// Create the request back
HttpWebRequest request =(HttpWebRequest)WebRequest.Create(requestUriString);

// Set values for the request back
request.Method = "POST";
request.ContentType = "application/x-www-form-urlencoded";
string obj2 = strFormValues + "&cmd=_notify-validate";
request.ContentLength = obj2.Length;

// Write the request back IPN strings
StreamWriter writer = new StreamWriter(request.GetRequestStream(), Encoding.ASCII);
writer.Write(RuntimeHelpers.GetObjectValue(obj2));
writer.Close();

//send the request, read the response
HttpWebResponse response = (HttpWebResponse)request.GetResponse();
Stream responseStream = response.GetResponseStream();
Encoding encoding = Encoding.GetEncoding("utf-8");
StreamReader reader = new StreamReader(responseStream, encoding);

// Reads 256 characters at a time.
char[] buffer = new char[0x101];
int length = reader.Read(buffer, 0, 0x100);
while (length > 0)
{
  // Dumps the 256 characters to a string
  string IPNResponse = new string(buffer, 0, length);
  length = reader.Read(buffer, 0, 0x100);
  string pay_stat = Request.QueryString["st"];
  //if (String.Compare(IPNResponse, "VERIFIED", false) == 0)
  if (!string.IsNullOrEmpty(pay_stat) && pay_stat == "Completed")
  {
     resultLabel.Text = "Grazie per aver effettuato il pagamento!"; +
  }
  else
  {
     resultLabel.Text = "Il pagamento tramite PayPal non è andato a buon fine.";}
}

//split iniziale per separare le varie stringhe in base al carattere '&'
String[] StringArray = strResponse.Split('&');

 int i;
            for (i = 0; i < StringArray.Length - 1; i++)
            {
                String[] StringArray1 = StringArray[i].Split('=');
                sKey = StringArray1[0];
                sValue = StringArray1[1];
                
                    switch (sKey)
                    {
                        case "first_name":
                            fname = sValue;
                            break;
...
            

<input type="hidden" name="return" value="http://www.tuosito.xx/tuapaginaRicevente.aspx"/> 

 
// See following url for more IPN code samples
        // https://cms.paypal.com/us/cgi-bin/?&cmd=_render-content&content_ID=developer/library_code_ipn_code_samples

        string postUrl = ConfigurationManager.AppSettings["PayPalSubmitUrl"];

        // Step 1a: Modifica la POST string.
        string formPostData = "cmd = _notify-validate";
        foreach (string postKey in Request.Form)
        {
            string postValue = Encode(Request.Form[postKey]);
            formPostData += string.Format("&{0}={1}", postKey, postValue);
            string logPathDir = ResolveUrl("Messages");
            string logPath = string.Format("{0}\\{1}.txt", Server.MapPath(logPathDir), DateTime.Now.Ticks);
            File.WriteAllText(logPath, formPostData);
        }

 WebClient client = new WebClient();
        client.Headers.Add("Content-Type","application/x-www-form-urlencoded");
        byte[] postByteArray = Encoding.ASCII.GetBytes(formPostData);
        byte[] responseArray = client.UploadData(postUrl, "POST", postByteArray);
        string response = Encoding.ASCII.GetString(responseArray);

 if (response == "VERIFIED")
        {
            //esegui le azioni di cui hai bisogno.
            
}
        else if (response == "INVALID")
        {
            //esegui le azioni di cui hai bisogno.
        }