It's easy to do an arbitrage chain a couple times, but people will start to get suspicious fast.
Suspicious? Sure, but will they care? He is dealing with Gringotts, or more specifically with specific goblins at Gringotts. They will be following their job specifications, probably their legal obligation, giving their company a profit and adhering to tradition. Gringotts wins, Harry wins, law is followed.
The people who lose are anyone who has invested in wizard cash (which is being inflated). But they aren't involved in the transaction and don't lose enough or rapidly enough that they would object before he has finished farming. In fact they only start experiencing negative effects once Harry starts spending.
What I'm more worried about are the implications on the Muggle side, and the fact it could endanger (even if lightly) the Statute of Secrecy. That is likely to draw political troubles from the wizarding world, and may be very well ruled illegal.
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 81, which should be published later today. The previous thread passed 400 comments as of the time of this writing, so it will pass 500 comments soon after the next chapter is posted, if not before. I suggest refraining from commenting here until chapter 81 is posted; comment in the 12th thread until you read chapter 81. After chapter 81 is posted, I suggest all discussion of previous guesses be kept here, with links to comments in the previous thread.
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.) When posted, chapter 81 should appear here.
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: one, two, three, four, five, six, seven, eight,nine, ten, eleven, twelve.
As a reminder, it’s often useful to start your comment by indicating which chapter you are commenting on.
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: