I looked into this a while back, but bailed on the idea because it violates the TOS of a bunch of APIs. For example, the Google API terms specifically notes that you cannot aggregate their content[1]. There were a number of issues with other API terms as well, such as completely disallowing any caching or storing of data, I can't recall which ones, but if you read the terms, you'll find they're quite restrictive.

Ultimately I decided not to pursue the idea since there's no real opportunity for growth, since all your content comes from your competitors, who can easily shut you down.

[1] https://developers.google.com/maps/terms, section 10.5 (d).