RealSelect Inc., a Thousand Oaks company that boasts the largest number of for-sale home listings on the Internet, has agreed to acquire a premier provider of online apartment listings. RealSelect, whose Realtor.com Web site has 1.25 million home listings, is acquiring San Francisco-based SpringStreet Inc., which has 6 million apartment listings. Lawrence Schoeffler, vice president of Best Image Marketing, an Internet services firm that caters to the real estate market, said RealSelect is the hands-down leader in home listings, and its acquisition of SpringStreet will make it the online market leader in both categories. “It’s an obvious move,” said Schoeffler. Officials with the privately held companies would not disclose the purchase price, but they said the all-stock transaction is expected to be finalized in the coming weeks. David Rosenblatt, vice president of marketing for RealSelect, said his employer will retain the SpringStreet name and integrate the apartment listings into RealSelect’s stable of other Internet offerings. John Helm, CEO of SpringStreet, will stay on to help integrate SpringStreet as a subsidiary of RealSelect. In addition to Realtor.com, RealSelect operates HomeBuilders.com, which lists 12,800 builders of new homes, and CommercialSource.com, an international listing of for-sale and for-lease commercial properties. “Now we can satisfy people’s housing needs during their lifetime,” said Rosenblatt. “From renting a house to buying a home to constructing a new home.” Founded in 1996 by Stuart Wolff, who remains chairman and chief executive, and Richard Janssen, a current board member, RealSelect derives its revenue from selling advertising on its Web site. It faces stiff competition in the race to dominate online real estate listings, with the most powerful competitor being Microsoft Corp. and its HomeAdvisor service. But RealSelect got a big jump on Microsoft and its other competitors by affiliating early on with the National Association of Realtors. As the official Web site of the NAR, RealSelect gained immediate access to the association’s members and listings, giving it a lead that its competitors have been unable to match, said Schoeffler. “RealSelect was the firstest with the mostest in this business, and unless they do something really dumb they’re going to continue being the market leader,” he said. The company has staked out similar turf with homebuilders by having its site designated as the official Web site of the National Association of Home Builders.