Twitter account for sale? Is it allowed?

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Does selling twitter account is allowed? I google it and I found some results. Andrew Baron of Rocketboom is sol his twitter account and followers in an auction in ebay. But there is one site who received Cease and Desist letter from twitter for they sell twitter accounts. What do you think about this? If it is allowed, do you know some marketplace for it? If not allowed then why some big companies acquired some twitter accounts (like MSNBC news acquired the @BreakingNews twitter account with 1.4million followers)?