Complex sentence: Little is known about Rosenberg's life prior to the 1880s; he lived in San Francisco in the 1880s, and was seen as an eccentric peddler.
Split into simpler sentences: Little is known about Rosenberg's early life; details about his life prior to the 1880s are few. While living in San Francisco in the 1880s, Rosenberg was seen as an eccentric peddler.