Witherspoon and Hello Sunshine’s senior management team will continue to oversee day-to-day operations. : Reese Witherspoon sold a majority stake in her production company to a new firm run by former top Walt Disney Co. executives and backed by the private equity firm Blackstone Group Inc., a deal that highlights the hot market for films and TV shows. The purchase, led by former Disney executives Kevin Mayer and Tom Staggs, values Witherspoon’s Hello Sunshine at about $900 million, according to a person with knowledge of the matter who asked not to be identified.