Buying/selling accounts are not contrary to Bitcointalk rules, but it is untrustworthy behavior.
18. Having multiple accounts and account sales are allowed, but account sales are discouraged.
After applying the merit system, selling accounts has become unprofitable because you need Merits that require high-quality posts. "If you can make high-quality posts, you don't need to buy an account."
Generally, many who sell accounts use new accounts so blocking them won't be a problem for them.