gistfile1.txt

<?php

    $args = array(
			'post_type'			=> 'property',
			'post_status' 			=> 'publish',
			'nopaging' 		        => true,
			'orderby' 			=> 'rand',
			'meta_query'			=> array(
				array(
					'key' 		=> '_on_market',
					'value' 	=> 'yes',
				),
				array(
					'key' 		=> '_featured',
					'value' 	=> 'yes'
				)
			)
    );
    
    $featured_property_ids = array();
    
    $properties = new WP_Query( $args );
    
    if ( $properties->have_posts() ) :
    
        while ( $properties->have_posts() ) : 
            
            $properties->the_post();
            
            $featured_property_ids[] = $post->ID;
        
        endwhile; // end of the loop.
    
    endif;
    
    wp_reset_postdata();
    
    // Not sure on the actual format of the [rev_slider] shortcode, but the key bit is the 'ids' bit.
    // Note: apparently you need 0 as the first ID
    // Ref: https://www.themepunch.com/faq/use-custom-wp-queries-post-based-sliders/
    echo do_shortcode( '[rev_slider alias="tester"]' . 
						   '[gallery ids="0,' . implode( ",", $featured_property_ids ).'"]' . 
						'[/rev_slider]' );

?>