add_filter('woocommerce_product_query','custom_pre_get_posts' ,99,2 );

function custom_pre_get_posts($q){
	if ( ! is_admin() && $q->is_main_query() && $q->is_search() && isset ( $q->query['only_active_auctions']) ){

		$metaquery = $q->get('meta_query');
		$metaquery []= 
			        array(
			            
			            'key' => '_auction_started',
			            'compare'=> 'NOT EXISTS',
			            	        
			    		);
		$metaquery []= 
			        array(
			            
			            'key' => '_auction_closed',
			            'compare'=> 'NOT EXISTS'				        
			    		);	        
		$q->set( 'meta_query', $metaquery );

		return;
	}
	return;
}