If this is true and those currently using the accounts are going to be held to account for the plagiarism of the farmer, we might as well ban account sales, even if you cant prevent all of them. I dont think it is reasonable to expect someone to check for plagiarism when they buy an account, or even know how to effectively check. This way if someone is banned for plagiarism the farmer did, there wouldnt be an injustice because they should have been banned for using the purchased account anyway (or the purchased account should have been locked).
I dont think it would be difficult to generally tell if a person in good standing had purchased their account after the time of plagiarism.
While I'm not opposed to account sales being banned. This line of thought might become peoples next attempt at a get out of Jail free card. Even if they were banned moving forward it wouldn't change where we are now in dealing with these past infractions. They bough the account and if they weren't going to do their part to ensure the reputation they were buying was clean they can deal with the consequences of it. See it can go both ways, they bough the account and rank for the benefits, they can deal with the negatives as well, I don't see an injustice here. Buyer beware and whatnot.
On that note, seeing as these appeals can be made to Moderators, they can admit to their account being a bought account without further repercussions, such as negative feedback depending on when it was purchased. In case they bought it after it became an unacceptable practice, as it's still allowed based on forum guidelines.