Thank you sir! That appears to work with some basic testing. Very much appreciated.
For the record, this is the final code I used, changing the error message to be more approprate:
PHP Code:
if($vbulletin->userinfo['posts'] < 30 AND $vbulletin->userinfo['joindate'] < TIMENOW - (86400 * 30))
{
$errormessage = "You do not have permission to create a thread in the Personal Marketplace because you need a post count of at least 30 and to have been a member for at least 30 days to create a thread here.";
eval('print_output("' . fetch_template('STANDARD_ERROR') . '");');
}