Don't buy or otherwise acquire (e.g. as collateral) Bitcointalk account(s). Don't sell or otherwise let others use your Bitcointalk account(s).
If you acquire an account you can be held responsible for any actions of the previous owner(s), and if you let others use an account you can be held responsible for their actions as well, including scams and bannable offenses. You're likely to get red trust for the purchase/sale as it is considered a deceptive practice similar to fake documents IRL.
Added! Great input. I think there were a lot of trustworthy members accepting collateral for Bitcointalk accounts because they were relying on people valuing that account more than the loan requested but its a poor business decision unless you intend to offload said account. If you don't and the loan is defaulted then the collateral meant nothing and you've just lost out on that. I agree that this should no longer be an acceptable thing to do.
TBH I'm not really sure if you can list every possible offense. How about loan requests without proper collateral, newbie PayPal trades, etc.
And even if you can - are the people who do these things going to read this and comply?
They probably aren't gong to read it if they intend to scam etc. But the list wouldn't be complete without listing that. But this is more intended to those that aren't clear of things that aren't immediately obvious. The collateral thing that you pitched in with for example isn't a clear and defined thing. That is something that many escrows and other trustworthy members have participated in.
I probably can't list everything but I'm going to try and do it anyway. If it benefits a few people then its useful.