# according to your example:
$categories = array ( \'sit\', \'leather\', \'one person\'  );

# get all id\'s by the category names
foreach ($categories as $category_name) {
$category = get_term_by( \'name\', $category_name , \'category\' ); 
$ids[] = $category->term_id;
}

# get all posts for the categories
query_posts( \'cat=\'.join(\',\',$ids) );