If you're getting your data free from Yahoo (scraping or API), aren't you limited by their non-commercial use clauses? That is, if you try to make money off of it without buying the data, doesn't that open you up to a huge liability risk?

Pretty sure that's stipulated all throughout their service (that you may not use any of their data for commercial purposes without their permission).

Acquiring that kind of data for commercial purposes normally costs a lot of money (relatively speaking).