cannot transform auction to regular product without hack

---- HOPE THAT EDIT DISPLAYS SYNTHAX HIGHLIGHTING (CODE INSIDE ----

Hi,

if I try to transform my auction at hook_form_submit(); by

db_query("DELETE FROM `uc_auction` WHERE `nid` = '%s'", $nid);

so that's not possible. I've written a hack with a $_SESSION-var to delete the auction in hook_node_api();

function alter_product_form_nodeapi(&$node, $op, $a3 = NULL, $a4 = NULL) {
    
   // auction deletion hack
    if(isset($_SESSION['check_delete'])){
    
    	$form_state['values']['nid'] = $_SESSION['check_delete'];
        // delete auction if one exists
        $rez = db_query("SELECT `nid` FROM `uc_auction` WHERE `nid` = '%s'", $form_state['values']['nid']);
		$res = db_fetch_object($rez);
		
		
		if(!empty($res->nid)){
			db_query("DELETE FROM `uc_auction` WHERE `nid` = '%s'", $res->nid);
		} else {
			unset($_SESSION['check_delete']);
		}
		
		$rez = db_query("SELECT `nid` FROM `uc_auction_bids` WHERE `nid` = '%s'", $form_state['values']['nid']);
		$res = db_fetch_object($rez);
		if(!empty($res->nid)){
			db_query("DELETE FROM `uc_auction_bids` WHERE `nid` = '%s'", $form_state['values']['nid']);
		}
		
		$rez = db_query("SELECT `nid` FROM `uc_auction_now` WHERE `nid` = '%s'", $form_state['values']['nid']);
		$res = db_fetch_object($rez);
		if(!empty($res->nid)){
			db_query("DELETE FROM `uc_auction_now` WHERE `nid` = '%s'", $form_state['values']['nid']);
		}
		
    }

$_SESSION['check_delete'] is defined in hook_form_submit():

function alter_product_form_submit($form, &$form_state) {


    if($form['#post']["is_auction"] == NULL){
    	
        $type = array(
          '0' => array('field_product_type' => 'Sofortkauf')
        );
       
       $_SESSION['check_delete'] = $form_state['values']['nid'];
       
	 
	 
    }

}

it's a hack. but what's the drupal-way for deleting the auction?