Yes. This is true even if account sales were somehow 100% eradicated as you would still have the possibility of hacked accounts. A signed message is also necessary to prove the terms of a trade with someone to the community in the event of a dispute. (PM's can also be used however it is more difficult and slower to prove content contained in a PM.
But a person would not NORMALLY go about asking the person to prove he owns the account while having an exchange/trade. Nobody till date has asked me for the same nor have I seen in other sections of this forum that anyone asks for a signed message openly while trading with them.
Hacking happens now as well and there would be such a possibility whether or not the rules are implemented.
No, you are using rape/murder's lack of 100% enforcement as justification for your point that even if it is unenforcable, you should try. You say, well rapes and murders aren't 100% solved, but there are still laws! Which is 100% incorrect on all levels.
Are you a law abiding citizen? Are you saying that the laws are incorrect at all levels? Sorry if that's the case, it wouldn't be wise enough to tell you to implement rules that can help members from getting scammed.
From all the above arguments, I cannot see how the forum is benefiting from these rules of account selling and creating multiple accounts. These rules are basically helping members to earn/scam other members. If everyone is happy with that, I cannot help but pray for those members who are going to be victims of such scams. More cases like this one
https://bitcointalk.org/index.php?topic=936429.0 (where the accused had sold his green trust account which was later used to scam a well known scammer) might be reported.