Trying to make php run in a post

Here’s a solution using a [shortcode] which doesn’t rely on running PHP inside of the content area. Here are the docs for get_post_meta() which show how that function should be called.

add_shortcode( 'buy_status', 'wpse249289_buy_status' );
function wpse249289_buy_status( $atts ) {
    $output="";
    if ( ! get_post_meta( get_the_ID(), 'buy_status', true ) ) {
        // Post meta value for key buy_status is false
        $output .= '<h5 style="text-align: center;display: block;">';
        $output .= 'Sorry, this piece is sold.';
        $output .= '</h5>';
    } else {
        // Post meta value for key buy_status is not false
        $output .= '<h5 style="text-align: center;display: none;"></h5>'; // Not sure why you'd want to output anything
    }

    return $output;
}