The Harry Potter book series and film franchise, worth well over $21 billion, continues to attract avid fans--even as many as nineteen years after author J.K. Rowling penned the first novel.  

On Tuesday, Universal Studios Hollywood announced that opening day tickets (Thursday, April 17) for the 'Wizarding World of Harry Potter' theme park in California had sold out. For the first time ever, the company had to halt online transactions. Premium ticket options, such as the "VIP Experience" and "Front Line" deal, also sold out for Thursday, as well as for Friday and Saturday.

"The incredible consumer demand speaks largely to the arrival of 'The Wizarding World of Harry Potter,' and the dramatic 75 percent transformation of Universal Studios Hollywood into an entirely new destination," said Larry Kurzweil, the president of Universal Studios Hollywood. 

It's worth noting that tickets sold out even as Universal Studios had introduced considerable ($20) price hikes last month (from $95 to as much as $115 for a standard ticket). It has also rolled out "surge pricing," à la Uber, which makes it more expensive to visit the park on peak days. 

The takeaway: If you build a strong, recognizable brand, and continue to launch new products periodically, you may never have to spend a dime on marketing again. 

It helps that J.K. Rowling herself is a master marketer who understands the difference between tasteful self-promotion and blatant advertising.

"The cool thing about J.K. Rowling is that she isn't advertising. She's not hitting people over the head with things they don't want to know," said Libby Turner, a director with the digital marketing agency Room 214, who has analyzed how Rowling conducts her social media accounts.

Rowling, who joined Twitter in 2009, has posted over 3,000 tweets and has amassed nearly 7 million followers--and counting. She tweets frequently about Harry Potter, in addition to news about her upcoming projects, like the play Harry Potter and the Cursed Child, and the film Fantastic Beasts and Where to Find Them.

"This is really a case study for how you can build a world, build a story, build a place where people really want to spend time and engage with you," Turner said.