Selling or buying accounts is considered a wrong act and if anything like this is seen by the moderators or Default Trust users both, seller and buyer are tagged with negative trust.
Technically, it is not prohibited to sell or buy accounts as it is not mentioned that way in forum rules, but it is not allowed because people buy high rank accounts and use them to harm others as those accounts are more trusted than smaller ones like Newbie or Jr. member. That is why actions are taken against buyers and sellers of bitcointalk accounts.