Have you check the database connection is ok? database name and table name too!
What error you get? or simply no results show? If you get error, fix it, otherwise you may have miss out something (variables such as id, date etc) in your script to call the listings/ads. You may use wildcard (%) to search your database. For example:
[php]$keyword = "%".$_POST['keyword']."%";
$query = "SELECT * FROM table WHERE description LIKE '$keyword' LIMIT 30";[/php]