> overwhelming evidence that the challenged conduct resulted in lower prices in Amazon’s store

Shkreli made back all the money from his clients that he misappropriated (by gambling with Retrophin shares). He still went to jail for fraud.

That was a criminal charge, not a class-action, and very different rules apply. Shkreli also committed one set of frauds against one company which affected many people; Amazon engaged in separate anti-competitive acts against many different companies which affected nearly the whole country.