As far as I know, there is no leniency given for the, "my account was hacked, I bought the account, it was my brother on my account, etc etc" excuses. It may well be valid, but its far easier for the staff to just say, take care of your account, rather than taking on the duty of spending days playing detective to help out the 1 in 1000 that have a legitimate claim. Back when account buying/selling was a bit more acceptable, before account farmers became the nuisance they are today, checking over an account's posts was something everyone did before buying an account. If I recall, there was even a price tool that would tell you the quality of an account's posts and its value. That burden was always on the buyer, so maybe I'm not as sympathetic as I could be on the matter. Regardless, asking the moderators to cut you some slack is just going to result in tons of spammers asking for forgiveness with bogus reasons, so I can't see anyone wanting to set that precedent.
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.