Hola !!!!
Pagina 1
<html>
<head>
<title></title>
</head>
<body>
<?php
echo "<form name='sell_1' action='sell_2.php' method='POST'>";
echo "<select name='Sell'>";
echo "<option value=1>valor 1</option>";
echo "<option value=2>valor 2</option>";
echo "<option value=3>valor 3</option>";
echo "</select>";
echo "<center><br><input type='Submit' name='enviar' value='Aceptar'></center>";
echo "<form/>";
?>
</body>
</html>
pagina 2
<?php
$aa=$_POST['sell'];
if ($aa=1)
{
echo " <html>";
echo "<head>";
echo "<meta http-equiv=\"refresh\" content=\"2;URL=test_12x.php\">"; //pagina de la opcion 1
echo "<title></title>";
echo "</head>";
echo "<body>";
}
elseif($aa=2)
{
echo " <html>";
echo "<head>";
echo "<meta http-equiv=\"refresh\" content=\"2;URL=test_12x.php\">"; //pagina de la opcion 2
echo "<title></title>";
echo "</head>";
echo "<body>";
}
elseif($aa=3)
{
echo " <html>";
echo "<head>";
echo "<meta http-equiv=\"refresh\" content=\"2;URL=test_12x.php\">"; //pagina de la opcion 3
echo "<title></title>";
echo "</head>";
echo "<body>";
}
?>
</body>
</html>
esta es una de las muchas formas, espero te sirva
para darte una idea
Salu2 y Suerte