I think they don't want people tagged as banned because people would ask why they did it. They would prefer the people just looked like they dropped off the face of the earth so they don't have to take responsibility for it. There are ways scammers can leverage this fact as well, which is unfortunately allowed to continue because of this secretive stance.
Well, if privacy had to be protected whilst also protecting the buyer from getting an account with bans, the bans could be visible to the user themselves only.
This means that if someone else logs onto your account, they can see your bans, and this would work if the account seller goes first, or uses an escrow who logs in and checks.
However, it doesn't seem like any actions are going to be taken to further help account sellers. And I don't see any good reason for why the forums should actively assist account sellers. Allowing them to stay is enough.
The last few posts are getting off topic.
There are plenty of reasons legitimate users would want to know why an account was banned. I don't like account sellers either, but this is not about them, this is about everyone else being able to know if an account is actually banned or if the user just went AWOL.