if in category but only with post meta

It looks like you just have an operator precedence issue.

Your logic here will always return true if $stat == ‘Sold Stat’ regardless of the result of in_category(‘secret-category’).

Try this, the extra parenthesis will force the || statement to be evaluated separately:

    if (in_category('secret-category') && ($stat == 'NFS Stat' || $stat == 'Sold Stat')) {
        echo 'Do something here.';
    }