I think OgNasty expressed a valid point, also if the forum staff will put a rule don't you think a lot of users will 'bypass' it? This is the unique reason why buying & selling (or have more than one) forum account is not forbidden... because it is impossible to stop this practice.
You can answer my one question? What benefit does the forum get from multiple accounts (especially those accounts created using one IP address)? Any good reason why is it allowed?
Also regarding members going against the rule. Laws are as well broken. That means there shouldn't be any law? Saying that as well does not make sense. Rules can atleast TRY to stop it.