Issues with update hook in drupal 6
Did the purchase-module create and implement the entire purchase node type or did you create that in the admin system?

If you created it in the admin-system you can't use those functions. They are for node type modules (a rarer and rarer kind of module use these days).

You probably want hook_nodeapi. It's a common misconception.
http://api.drupal.org/api/drupal/developer%21hooks%21core.php/function/hook_nodeapi/6

function hook_nodeapi(&amp;$node, $op, $a3 = NULL, $a4 = NULL) {
  switch ($op) {
    case 'update':
      if($node-&gt;type == 'purchase') {
        echo 'Updating';
        exit(0);
      }
    break;
  }
}