This hook fires when the item is being updated / saved to the database.

/**
 * @param $inventory_id
 * @param $data
 */
function my_wpim_save_item($inventory_id, $data) {
    // Do whatever you want from here.  Such as sending to another table, update the owner of the item via an email, etc.
}

add_action( 'wpim_save_item', 'my_wpim_save_item', 10, 2 );