Second Measure (YC S15) Offers Live Data Analysis of Public/Private Companies
techcrunch.com
techcrunch.com
If we recall back a few months, analysts at Capital One were investigated and charged by the SEC related to insider trading of consumer stocks by examining very similar information streams.
http://www.bloombergview.com/articles/2015-01-23/capital-one...
Of course it is legal. Credit card companies have had agreements in place to share information like this for years. In some cases it isn't anonymized either. Acxiom[1] is probably the best known company working in this area.
[1] http://www.nytimes.com/2013/09/01/business/a-data-broker-off...
Especially as an invitation only "not public" offering.
Even then, it wasn't a clear-cut case.
If you are sourcing the data from other sources and don't have that kind of relationship it really isn't going to be a problem
http://www.wsj.com/articles/provider-of-personal-finance-too...
I spent many hours trying to understand how to grab that information to make our app more useful. Unfortunately, the best I could come up with was partnering with POS companies.
However, I can imagine how much it must cost to purchase the credit card data. I don't think credit card companies would offer that up for cheap.