Redfin Corp., a company that helps people buy homes online without a typical real-estate agent, is expected to announce that it has raised $8 million from venture-capital investors and is expanding its services from its home base of Seattle into California.The company's backers include Vulcan Capital, the Seattle private-investment group of Paul Allen's Vulcan Inc., as well as BEV Capital of Stamford, Conn. and Hillman Co., a Pittsburgh investment company. Seattle's Madrona Venture Group, which invested $750,000 in Redfin last year, also took part in the financing.