If you make the kind of money that allows you to start multiboxing, it's highly unlikely that you'd need to resort to selling accounts to supplement your income.

Twinks on the other hand are a completely different story. Because a huge number of them are kids, many don't have the kind of income to support even one account, so account selling and playing private servers is rampant among twinks.

Multiboxing implies a much higher level of maturity, in order to both manage the technical side of it, and support it financially. That maturity suggests that multiboxers are more responsible, and likely to follow the rules.