Plagiarism was neither explicitly against the rules when he bought the account nor was it a known problem. He should have known not to plagiarize himself (by all accounts he did not), but I don't think he had any reason to believe others were plagiarizing.
I dont think he had any reasonable reason to check for plagiarism when he bought it, and I dont think any of the tools that checked post quality would look into potential plagiarism.
As previously stated, if account buyers are going to be held responsible for the actions of prior owners, account sales might as well be disallowed.
It technically was, we just called it spam at the time as it was not common with non newbie accounts. Account farmers were a problem even back then, and plagiarism itself wasn't uncommon. Account farmers had hundreds of accounts and they'd share posts in megathreads and places they could get away with it. Those accounts were nuked/banned, but none of them ever appealed because they knew exactly what they were doing.
I don't know for certain, but I wouldn't be surprised if Bill's account was one that slipped through the cracks, and later got flagged by the bot.