Mage::registryはメソッド間で保持されません。考え?(はい、displayOrderが最初に呼び出されます)。
public function displayOrderAction()
{
$this->loadLayout();
$post = $this->getRequest()->getPost();
$block = $this->getLayout()->getBlock('returner.displayorder');
$returnOrder = Mage::getModel('returner/order')->getOrder($post['increment_id'], $post['email']);
Mage::register( 'returnOrder', $returnOrder );
$this->renderLayout();
}
public function verifyItemsAction()
{
$post = $this->getRequest()->getPost();
$order = Mage::registry('returnOrder');
}