I don't know the particulars, so hard to say. You're probably wanting me to say intent has bearing. Madoff intended to scam anybody as long as it helped his situation along. Don't know what Stewart's intent was.

You buying positive reviews is intended to make more money for yourself by getting as many sales as possible including if it means another seller doesn't get that sale. The person buying negative reviews is intending to take sales from others so they get the sales instead. Either way, the intent in both situations is to increase one's own position at the expense of others in a fraudulent manner. You're both just as guilty.