Except that observed information can't be changed using the Time-Turners. So the scope of his actions are somewhat limited. Doesn't stop him from being able to short a stock, but he can't single-handedly cause a stock's fortune to reverse. That still leaves plenty of possibilities to make money, but it wouldn't be as easy as it sounds. He'd be mixing the complicated natures of stock trading and time travel, and that's before he starts thinking about avoiding insider trading laws.
It's pretty easy, isn't it?
Check the exchange status, go back in time, buy stocks that are about to go up in price. He can't stop them from going up in price, sure, but him buying is perfectly consistent. I expect he could get a few percent per day without even trying hard.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 97. The previous thread is at nearly 500 comments.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system.
Also: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: