As an Aquarius, David Birnbaum is naturally skeptical of astrology. But as an investor, he has zero doubts about the business potential of the $2.1 billion “mystical services market.” It’s an area that he has been trying, unsuccessfully, to invest in for nearly two decades. Mr. Birnbaum researched lots of astrology start-ups in the Web 1.0 era but concluded then that they were not good investments. “They were pretty much just marketplaces sending traffic off to random astrologers,” he said.