If it is the teacher’s edition of a school book, that’s a longstanding policy. Otherwise, ???.
Yes, sounds like a copyright issue, not censorship. Blocking the email address seems odd unless there’s a history of some sort.
Time to leave eBay. I left after 17 years after eBay would not make good on their stated guarantee and allowed a seller to send me dead plants when I paid for live ones and wanted me to pay postage for replacements. EBay is guilty of many cases of fraud. They also delete negative feedbacks to improve the feedback score of their “super sellers”, as they get a cut out of every sale.