If you want to trust somebody it would be better to use things like gpg signed contracts and the OTC-WOT than just relying on a forum account being who you think they are.
I buy GPG keys too, we will essentially buy your whole online identity.
There are uses of buying accounts other than scamming, in fact, highly reputable accounts are rarely sold for scamming, people are willing to pay premium for them so that they can use them to run their legitimate businesses/websites, it makes more sense to do that then to scam for a few bitcents. I also refuse to do business with scammers although other traders don't care.
Why does the forum allow the account selling threads? Well because account trading happens in private all the time too, there is nothing anyone can do to stop that.
I was the first person to start selling accounts on this forum publicly, my actual intentions were to highlight the fact that this can happen and does happen and move people away from trusting post counts and reputation and to use 2 of 3 escrows etc and it's worked. Accounts can be hacked or purchased and people need to realize that rep means nothing, trust no one.