are buying and selling of forum accounts permitted.and what is the benefit of buying another account when you already have an account
It is permitted but frowned upon as the buyer can use the bought account if it has enough trust on it for scamming.
Benefits are that you can participate in signature campaigns and conduct trades in goods or services for bitcoin.
Senior and above ranked accounts make the most since it takes you up to 6 months to get that status if you post almost everyday(actually once per week) to gain the activity points that are given on a biweekly basis.