Buying and selling of accounts facilitate spam, shitposting and account farming. They are marked with a red tag in case they are caught. Sold accounts are the biggest source of scams happening and then the shiposters are always there.
RE: Invites & Accounts section, This following quote should give you a clear idea about the purpose of this board.
Its not about being legal or illegal, but its a question of morals. Its morally wrong to sell something which can cause huge loss for the community later. Also that section was for other accounts not bitcointalk.org accounts. Majority of accounts sold are just for the purposes of spamming the forum through signature campaigns and the accounts sold with positive trusts are used for scamming people, its that simple. So i think people deserve the negative trust for doing something immoral.