Since there are quality differences between data providers, it would be good to know where you source the data from. Is it from Yahoo Finance (just for example)? Or is it really directly from Nasdaq (which I presume would be pretty costly, and thus raises the question on how long you'll be able to run this service for free before shutting down or having to raise prices).

This is important since if someone decides to fully build on top of your API, they don't want the service to be gone in a few months and waste all the time integrating with yours.