Ask HN: Legal concerns for aggregating financial transactions

I've been playing around with the Plaid Transactions API. I think it would be pretty interesting to offer a product similar to Mint.com that allows for more slicing and dicing of your data.

While I'm having fun playing with my own data, do any HNers know of legal/privacy issues I might run into if I start storing other people's transactions?

I work in the healthcare space and am used to being very careful with PII like this. Are there any regulations like HIPPA that I should be mindful of?

Edit: I am in the US