Opendoor, a San Francisco startup that buys and sells homes, has raised $325 million in new funding led by General Atlantic, Access Technology Ventures, and home-builder Lennar. Bottom line: Opendoor says that it's on track to purchase more than $2.5 billion worth of homes this year — up from a $1.3 billion run rate last October. But it doesn't disclose data on homes sold, and Opendoor's ability to profitably offload is viewed as a major risk factor.