MySQLでSUMの最大値を見つける

これを試してください：

$test = "SELECT c_id, MAX(Bought) AS MaxBought FROM (SELECT c_id, SUM(n_sell) AS Bought FROM sell GROUP BY c_id) AS tmp HAVING MAX(Bought) = tmp.Bought"; 
$resut = mysql_query($test) or die(mysql_error());
while($t = mysql_fetch_array($resut)){
    echo "Number of sold: ". $t['MaxBought'] ." to". $t['c_id'] .":id of customer";
    echo "<br />";
}

理解を容易にするためのSQLクエリのみを次に示します。

SELECT c_id, MAX(Bought) AS MaxBought
FROM (SELECT c_id, SUM(n_sell) AS Bought
      FROM sell
      GROUP BY c_id) AS tmp
HAVING MAX(Bought) = tmp.Bought