The Unruh act does not attempt to make illegal every interaction between individuals. A law that attempted to prohibit private people from discrimination would run afoul of the first amendment. The Constitution guarantees your right to be wrong! Americans have a constitutional right to hold racist, sexist, homophobic views.
The Unruh act instead only attempts to regulate your interactions with businesses. When you interact with a business you have a right to be treated fairly:
All persons within the jurisdiction of this state are free and equal, and no matter what their sex, race, color, religion, ancestry, national origin, disability, medical condition, genetic information, marital status, sexual orientation, citizenship, primary language, or immigration status are entitled to the full and equal accommodations, advantages, facilities, privileges, or services in all business establishments of every kind whatsoever.
A property sale is normally a private transaction between two people. The realtor a "business establishment" and is subject to the act. The Seller is not a business establishment and is not subject to the act.
The act doesn't exclude the sale of buildings and properties. But it does exclude private transactions. The Unruh act does not apply to (for example) private sales of automobiles. But it does apply to sales of property by development companies.