...
If selling accounts was frowned upon or discouraged, more people would leave account sellers negative feedback. ...
From the unofficial list of unofficial not rules but suggestions to be interpreted however you wish:
...
18. Having multiple accounts and account sales are allowed, but account sales are discouraged.
...
mprep's forum guidelines reflect his personal opinion as well, hense the "unofficial" in the title. Account sales aren't discouraged by forum administration, because they don't care. If someone sells a trusted account, the chances of that account losing its trust are equally high.