Don't do that... Retaliatory feedbacks always result in more negative feedbacks. Not to mention that your feedback is marked as "untrusted" and doesn't count for anyone's trust score. So, you're just attracting more negative feedbacks.

But it doesn't really matter. Account buyers/sellers usually get many negative feedbacks and they stay there. Buying accounts may not be against the forum rules, but it is very untrustworthy from the eyes of the community, so we all agree that the user deserves negative feedback. Your account is "useless" now (assuming that you only bought it to earn money).