private void Form1_Load(object sender, EventArgs e)
{
CookieContainer cookieJar = new CookieContainer();
HTTP http = new HTTP(cookieJar);
string user = "******";
string pass = "******";
//some cookies are set before login & they are needed for an user to login, for that reason only you were getting the error
string responseData = http.DownloadString("https://signin.ebay.com/ws/eBayISAPI.dll?SignIn");
string midParamValue = ParseMid(responseData, "name=\"mid\" id=\"mid\" value=\"", "\"");
string bUrlPrfx = ParseMid(responseData, "id=\"bUrlPrfx\" value=\"", "\"");
string rqid = ParseMid(responseData, "id=\"bUrlPrfx\" value=\"", "\"");
string hidUrl = ParseMid(responseData, "name=\"hidUrl\" value=\"", "\""); //"name=\"hidUrl\" value="http://my.ebay.com/ws/eBayISAPI.dll?MyeBay"
//string lucky9 = ParseMid(responseData,
string pdata =
string.Format(
"MfcISAPICommand=SignInWelcome&bhid=a1%253Dna%7Ea2%253Dna%7Ea3%253Dna%7Ea4%253DMozilla%7Ea5%253DNetscape%7Ea6%253D5.0%2520%28Windows%29%7Ea7%253D20100101%7Ea8%253Dna%7Ea9%253Dtrue%7Ea10%253DWindows%2520NT%25206.1%253B%2520WOW64%7Ea11%253Dtrue%7Ea12%253DWin32%7Ea13%253Dna%7Ea14%253DMozilla%252F5.0%2520%28Windows%2520NT%25206.1%253B%2520WOW64%253B%2520rv%253A18.0%29%2520Gecko%252F20100101%2520Firefox%252F18.0%7Ea15%253Dfalse%7Ea16%253Den-US%7Ea17%253Dna%7Ea18%253Dsignin.ebay.com%7Ea19%253Dna%7Ea20%253Dna%7Ea21%253Dna%7Ea22%253Dna%7Ea23%253D1366%7Ea24%253D768%7Ea25%253D24%7Ea26%253D738%7Ea27%253Dna%7Ea28%253DFri%2520Jan%252025%25202013%252022%253A20%253A09%2520GMT%252B0530%2520%28India%2520Standard%2520Time%29%7Ea29%253D5.5%7Ea30%253Dpdf%257Cpdf%257Cpdf%257C%7Ea31%253Dyes%7Ea32%253Dna%7Ea33%253Dna%7Ea34%253Dno%7Ea35%253Dno%7Ea36%253Dyes%7Ea37%253Dno%7Ea38%253Donline%7Ea39%253Dno%7Ea40%253DWindows%2520NT%25206.1%253B%2520WOW64%7Ea41%253Dno%7Ea42%253Dno%7Ea43%253D&UsingSSL=1&inputversion=2&lse=true&lsv=11.5.502&mid=AQAAAThpCRHvAAUxMzg5OTM0YzA2MS5hNWFiODMyLjJmNDcyLmZmZTlhOWUwEZMPFiRFo7yUjykHfCAFgp%2Bk9%2Bk*&kgver=1&kgupg=1&kgstate=r&omid=&hmid=&rhr=f&siteid=0&co_partnerId=2&ru=&pp=&pa1=&pa2=&pa3=&i1=-1&pageType=-1&rtmData=&bUrlPrfx={0}&rqid={1}&kgct=&userid={2}&pass={3}&keepMeSignInOption=1&sgnBt=Sign+in&htmid={4}&kdata=%251E%251F%2510%251E1359132609409%251E1%251F%2510%251E1359132609409%251E0%251F%2510%251E1359132609410%251E1%251F%2510%251E1359132609410%251E0%251F%251B%251E1359132612808%251E1%251F%2510%251E1359132612837%251E1%251F%2510%251E1359132612838%251E0%251F%251B%251E1359132612977%251E0%251F", bUrlPrfx, rqid, user, pass, midParamValue);
responseData =
http.UploadString("https://signin.ebay.com/ws/eBayISAPI.dll?co_partnerId=2&siteid=0&UsingSSL=1", pdata);
richTextBox1.Text = responseData;
PrintAllCookies(cookieJar);
}