There's one more thing I'm worried about. Suppose we sold our account. The person who bought the account did an illegal job with this account.
Is it possible that we are not guilty? Can they sue us for they know our IP address?
First of all your login address is visible in the database, so if someone buys your account he will use a different ip than you did.
If the possibility of him scamming worries you, you can always keep the records of the sale, dates, addresses, conversations and if someone comes knocking at your door you will be able to prove you had no control over the account since (date). Since selling account is not prohibited on the forum, you haven't violated any laws and can't be held responsible.