I need an addon which extends permissions so that certain user groups can, in specific forums, only reply to threads with a specific prefix. For example, we use thread prefixes to denote For Sale, Wanted and For Trade adverts. And we want to put a limit on new members so that they can not create new advert threads or reply to Wanted ads, but *can* reply to For Sale ads. I.e. they can buy, but not sell. So they can only reply to threads with the For Sale prefix. Is this tricky?