Manipulating the stock market isn’t hard if you aren’t ethical. Elon Musk did it a ton. From the killer AI standpoint, there are a few tricks, but generally create a bad news event for various companies and either invest while it is low and recovers when the news is found to be fake, or short it to time with the negative event. On top of that, a non-ethical super intelligence could likely hack into networks and get insider information for trading. When you discard all ethics, making money on the stock market is easy. It works well for congress.