Firstly, you aren't able to sell an account on the website, only a character. Though they have made a vague statement about account trading/sharing they still do not condone it. They have stated though that an account punished for account trading in the past will not change because what they did was against the rules at the time the person chose to do it.
"I have received a punishment for account trading/sharing in the past. Will I get compensated?
No, because a Tibia Rule was violated at the point of time when the punishment was issued. Moreover, account trading and sharing still goes against our Service Agreements and we do not condone it nor do we provide any kind of support in case problems arise as a consequence."
Source: https://www.tibia.com/news/?subtopic=latestnews&id=5692#faq3