CSS conditional on custom field value

Without any other context (which means you will likely need to adapt to your specific needs), I would consider using the post_class filter. Using this filter, you can access the post meta for the current post, and conditionally add a CSS class to the post container.

For example:

function wpse129167_filter_post_class( $classes, $class, $postid ) {
    // Get post meta value for fl_status
    $fl_status = get_post_meta( $postid, 'fl_status', true );
    // If fl_status = sold, add a CSS class
    if ( 'sold' == $fl_status ) {
        $classes[] = 'fl-status-sold';
    }
    // Don't forget to return the array
    return $classes;
}
add_filter( 'post_class', 'wpse129167_filter_post_class', 10, 3 );

Look at your post markup inside your loop. Look for this:

<div id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
    <!-- post markup will be here
</div>

For posts with a post meta key fl_status with a value of sold, that call to <?php post_class(); ?> will, among the usual classes, output the class fl-status-sold.

So, you can target this class with your CSS (directly in style.css, if you wish):

.fl-status-sold #firstimg {
    opacity:0.4;
    filter:alpha(opacity=40);
}