It's not prohibited because people are also allowed to have multiple accounts so long as they don't have any ill intentions.

But for the community where there are lots of business deals,Loans, trades etc, a person who engages in account sales is not a person to be trusted that why you receive the negative tags... to warn other people that the account has changed hands and so when dealing with the account, one should proceed with extreme caution. The previous owner may have been trustworthy but nobody knows if the new owner is trustworthy or not. That's the whole sense of getting the sold account tagged.