What we have? The guy bought an account. Has not violated any rules of the forum.
He did violate the rules of the forum - by plagiarizing.
He was not banned for buying an account.
Apparently you have not carefully read. User account, has not violated any rules. Previous owner, guilty of plagiarism. Plagiarism was discovered after the sale of the account. Sorry, but there is clearly no understanding of the situation.
If in fact, he bought already means his account, it means you had to look in advance whether there is a problem on that account or not.and not give money, not knowing what with this account.