I think I figured it out after looking a little harder at some of @kory’s posts.
Here’s what I did:
$query = array(
  'post_type'           => 'listing’,
  'post_status'         => 'publish',
  'orderby'             => 'date',
  'order'               => 'DESC',
  'meta_query'           => array(
    'relation' => 'AND',
    array(
      'key' => 'ffd_status',
      'value'    => 'Closed',
      'operator' => 'NOT IN',
    ),
    array(
      'key' => 'ffd_listingtype',
      'value'    => 'Sale',
      'operator' => 'IN',
    ),
  ),
);
var_dump( http_build_query( $query ) );
Here’s the output:
post_type=listing&post_status=publish&orderby=date&order=DESC&meta_query%5Brelation%5D=AND&meta_query%5B0%5D%5Bkey%5D=ffd_status&meta_query%5B0%5D%5Bvalue%5D=Closed&meta_query%5B0%5D%5Boperator%5D=NOT+IN&meta_query%5B1%5D%5Bkey%5D=ffd_listingtype&meta_query%5B1%5D%5Bvalue%5D=Sale&meta_query%5B1%5D%5Boperator%5D=IN