[kh99]

Yes, the rule is that the user has the maximum permission allowed by any group the belong to. From the vbulletin manual:

The basic idea here is that if a user is in multiple groups and they specify conflicting permissions, the greater permission overrides. Thus, a yes will always override a no and a larger number will override a smaller number.
Note:
There is a slight exception to this rule. If 0 represents unlimited or no restriction, then it will override any other setting; it really is the greater permission.

So maybe you could reverse things and add everyone to a secondary group that gives permissions to the classifieds.