Hi,
Our API does include results for events that aren't ticketed by us. That said, it is limited. Most of our partners ticket their events exclusively with us, but in certain instances, they might need to use a different ticketing provider for a particular event or group of events. Ticketfly offers many features before and after the transaction, so our clients still enter those events into the system so that they can take advantage of them.
While you can rely solely on Ticketfly's API, you will not get 100% coverage in any given area of North America.
Perhaps some of the other folks in the group who are aggregating multiple data sources can give you some insight into how they approach this.
Good luck with your site!
Thanks!
-b
Bill Rousseau | Product Manager | Ticketfly