1. Preferentially display bad news about a company
2. Buy a lot of their stock
3. Preferentially display good news about them
4. Sell stock

They could do this for a large number of companies at once. And randomize it so it wasn't obvious to others. This would be a variant of pump and dump. I don't know if it would be illegal, though it would definitely hurt them if it got out they were doing it.

Mostly this shows how vulnerable we are in treating google as the source for the relative importance of pages on a given topic.