Getting MLS data is hard. We had to become a registered California brokerage and join individual MLS organizations as participating agents. Each MLS runs on different servers with slightly different SOAP/XML APIs and slightly different data formats. We're ingesting from 3 right now in real-time. We're planning to get full CA coverage this way in the next month or so (CA is 15% of the national real estate market).
We might have a different approach when we scale nationally but so far this is working pretty well. There are a few third party companies that offer the data but it's not as fast or complete as a brokerage feed from the MLS (and speed matters in hot CA markets).
That said, we don't need the property in our system to put an offer on it so you can shop on Redfin or Zillow and come back to our site when you want to put a bid in. We will happily put an offer in on any home in California.