You can buy one if you wish as it's not against the forum rules but certain members will leave you negative for engaging in such behaviour if they deem it appropriate. Everybody starts as a newbie so why don't you just build up your account and reputation like pretty much everyone else did instead of trying to buy it.
Why? Because of the advantages. I can't even participate in signature campaigns?! It's like saying to someone leaving university to start with cleaning work or sweeping streets. 
But I understand that trust has nothing to do with the forum rules but everything with what users think of one another. Correct? Does that also mean that there are no forum rules, no sanctions, and that it's a wild West and anarchy around here?