[jacquesm]

> If you sell links you should be putting rel="nofollow" on them.

Is there a law that states that you should do that?

Is this 'best practice' without which clients no longer function?

[dools]

It's one of Google's guidelines. If you buy or sell links without doing that you risk being penalised by Google. Whether or not that matters to you depends on your site/business.