add_action( 'wp_loaded','my_flush_rules' );
    add_filter( 'rewrite_rules_array','my_insert_rewrite_rules' );
    add_filter( 'query_vars','my_insert_query_vars' );

    function my_flush_rules() {
        $rules = get_option( 'rewrite_rules' );

            if ( ! isset( $rules['(seller-login)/(.+)$'] ) ) {
                global $wp_rewrite;
                $wp_rewrite->flush_rules();
            }

    }

    function my_insert_rewrite_rules( $rules ) {
        $newrules = array();
        $newrules['(seller-login)/(d*)$'] = 'index.php?pagename=$matches[1]';
        // var_dump($newrules);
        return $newrules + $rules;
    }

    function my_insert_query_vars( $vars ) {

        array_push($vars, 'pagename');

        return $vars;
    }