By default, users are required to register before they can post an ad listing, so if you wanted to disable this it would require customization on your part as there is no setting to turn it off in the ClassiPress admin dashboard.
Although you can create various ad packages, you can't specify which users can purchase/use them. To allow free ads for some, you could make use of coupons instead via the
AppThemes coupons plugin.
There is no store front or invoicing option available within the theme so this would require customization on your part. Thanks.