$url = 'https://steamcommunity.com/tradeoffer/' . $tradeID . '/accept';
$partnerID = '';
$tradeID = '';
$data = array(
'sessionid' => $sessionId,
'serverid' => '1',
'partner' => $partnerID,
'tradeofferid' => tradeID,
'captcha' => ''
);
$c = curl_init();
curl_setopt($c, CURLOPT_HEADER, false);
curl_setopt($c, CURLOPT_NOBODY, false);
curl_setopt($c, CURLOPT_URL, $url);
curl_setopt($c, CURLOPT_SSL_VERIFYHOST, 0);
curl_setopt($c, CURLOPT_USERAGENT, "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.0.3705; .NET CLR 1.1.4322)");
curl_setopt($c, CURLOPT_COOKIE, $cookies);
curl_setopt($c, CURLOPT_POST, 1);
curl_setopt($c, CURLOPT_POSTFIELDS, http_build_query($data));
curl_setopt($c, CURLOPT_RETURNTRANSFER, true);
curl_setopt($c, CURLOPT_HTTPHEADER, array('Referer: https://steamcommunity.com/tradeoffer/' . $tradeID));
curl_setopt($c, CURLOPT_SSL_VERIFYPEER, 0);
curl_setopt($c, CURLOPT_FOLLOWLOCATION, 1);
curl_setopt($c, CURLOPT_CUSTOMREQUEST, strtoupper('POST'));
$response = curl_exec($c);
curl_close($c);
echo $response;
$TradeID можно найти отправив Get запрос на http://api.steampowered.com/IEconService/GetTradeOffers/v1/?key=[API_KEY]&get_received_offers=1&time_historical_cutoff=100 ;
$sessionID и $cookies берем из авторизации.