public void addItem (ulong itemid, int slot)
{
var data = new NameValueCollection ();
data.Add ("sessionid", Uri.UnescapeDataString (sessionId));
data.Add ("appid", "440");
data.Add ("contextid", "2");
data.Add ("itemid", "" + itemid);
data.Add ("slot", "" + slot);
Fetch (baseTradeURL + "additem", "POST", data);
}