Как исправить ошибку в POST запросе

Хочу передать данные пост запросом к steam, но в ответ получаю ошибку, помогите разобраться:

 var request = (HttpWebRequest)WebRequest.Create("http://steamcommunity.com/market/createbuyorder/");
        var postData = "sessionid=ххх";
        postData += "&appid=730";
        postData += "&currency=5";
        postData += "&market_hash_name=USP-S | Guardian (Field-Tested)";
        postData += "&price_total=1";
        postData += "&quantity=1";
        var data = Encoding.ASCII.GetBytes(postData);
        request.Method = "POST";
        request.ContentType = "application/x-www-form-urlencoded";
        request.ContentLength = data.Length;
        using (var stream = request.GetRequestStream())
        {
            stream.Write(data, 0, data.Length);
        }
        var response = (HttpWebResponse)request.GetResponse(); // здесь выдает ошибку 400 Bad Request
        var responseString = new StreamReader(response.GetResponseStream()).ReadToEnd();

А вот, что приходит в коде с сайта при выполнении вручную, куки не писал по понятным причинам: General

Request URL: https://steamcommunity.com/market/createbuyorder/ Request Method: POST Status Code: 200 OK Remote Address: 104.76.35.111:443 Referrer Policy: no-referrer-when-downgrade

Response Headers

Access-Control-Allow-Credentials: true Access-Control-Allow-Methods: POST, GET, HEAD, OPTIONS Access-Control-Allow-Origin: https://steamcommunity.com Access-Control-Expose-Headers: X-NotLoggedIn Access-Control-Max-Age: 604800 Cache-Control: no-cache Connection: keep-alive Content-Length: 40 Content-Security-Policy: default-src blob: data: https: 'unsafe-inline' 'unsafe-eval'; script-src 'self' 'unsafe-inline' 'unsafe-eval' Content-Type: application/json; charset=utf-8 Date: Wed, 19 Dec 2018 07:55:19 GMT Expires: Mon, 26 Jul 1997 05:00:00 GMT Server: nginx Vary: Accept-Encoding, Origin X-Frame-Options: SAMEORIGIN

Request Headers

Accept: / Accept-Encoding: gzip, deflate, br Accept-Language: ru-RU,ru;q=0.9,en-US;q=0.8,en;q=0.7 Connection: keep-alive Content-Length: 132 Content-Type: application/x-www-form-urlencoded; charset=UTF-8 Cookie: Host: steamcommunity.com Origin: https://steamcommunity.com Referer: https://steamcommunity.com/market/listings/730/AK-47%20%7C%20Redline%20%28Field-Tested%29 User-Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36

Formd Data

sessionid: 3b5608538832293729121e4c currency: 5 appid: 730 market_hash_name: AK-47 | Redline (Field-Tested) price_total: 100 quantity: 1