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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.
Lets put it this way: if I purchase a car from some random guy I may be an angel driving and looking after the car, always complying with regulations and keeping it bright and shiny. Nevertheless, the car and its previous owner may have a history that spins back on me even though I have nothing to do with it.
For example, the car might have been used as collateral in a loan, or simply be on the list of claims from the tax office for the previous owners debts. It can also have unforeseen internal pathologies that spring to life, or have been used to commit a crime and be required as proof by the authorities.
These kind of risks come along with purchasing goods from another person, and in this case there is no way round but to accept the loss of the account.