Selling accounts is "allowed" here by the forum rules. Bitcointalk staff will not ban you for buying or selling accounts. The trust system and its feedback are completely independent from forum staff. It was decided by the majority of the community that account buying/selling is untrustworthy behavior, and for that reason, its not uncommon to receive negative feedback if you are caught doing it. Selling positive trust isn't against forum rules either, but its the same deal as before, people find it to be an untrustworthy behavior, so you will get negative feedback for doing it.

People seem to be under the impression that Default Trust (DT) is this uniform system that follows very strict rules and behaves much like the moderation staff here, but for trade related things. For the most part, its just each person's personal preferences as to whether they decide to give out negative feedback or not. There are some mutual similarities between people across the board, like account selling and trust trading, however there is no rule saying that a person on DT has to follow any specific guidelines, short of not doing something that gets them kicked off for being harmful.