Is it illegal to sell negative SEO?

[BrettUK]

The title says it all really, I have seen a few people who have been selling negative SEO services.

Is it illegal?

[BrettUK]

Yeah I agree.
At the end of the day, you're only building links.
Is link building illegal? 

[Web-Master]

Concentrating on doing negative seo to our competitors is waste of time and money, If they do the same thing for our website then it will also effect us. For every action there is equal and an opposite reaction....