Sid Zilber is no fan of Harry Potter. He has never read the enormously popular series of children's books about the junior wizard. He doesn't know a Muggle from a Martian or a Nimbus 2000 from a Nissan 300ZX.

But he does know a good idea.

"A great idea," the 53-year-old businessman and former lawyer says happily in his plush offices in the Philadelphia suburb of Bala Cynwyd.

Hoping to cash in on Pottermania, Zilber is selling sweat shirts, T-shirts and baseball caps with Potteresque logos on the Internet. Since launching his Web site a month or so ago--www.quidditchclothes.com--he has filled 150 orders.

"Right now we're thinking of expansion," says Zilber, who has stacks of black shirts and hats in a spare room in his offices, where he runs Commonwealth Insurance Co., a bonding company.

The caps are emblazoned with the word "Muggles," and the shirts with Hogwarts Quidditch Team Gryffindor logos with crossed brooms.

(In Potterese, a Muggle is a human, Hogwarts is a wizardry school, Quidditch is a game played with flying broomsticks and four balls, and Gryffindor is Harry's team at Hogwarts.)

For you Muggles who haven't read the books, Harry Potter is a publishing phenomenon. Children as well as adults can't get enough of Harry, a lovable, bespectacled young boy who enters a world of witchcraft and wizardry after his parents are killed.

Penned by Scottish author J.K. Rowling, the first three books--a total of seven are planned--have performed a literary hat trick, capturing the three top spots on the New York Times bestseller list.

Now, since anything that appeals to children is bound to be licensed and sold in various forms of merchandise by mass-market toy stores and fast-food restaurants, Zilber's Quidditchclothes enterprise begs the question: Can he do that?

According to Warner Bros., which owns the movie and merchandise rights: No. About two weeks ago the company sent Zilber a letter telling him to stop selling the clothes "and give them everything--the merchandise, the Web site," he said. The letter, he said, also contained "a bunch of technical stuff about copyrights, etc."

The ponytailed entrepreneur maintains he's done nothing wrong.

After getting the idea for the clothes from a friend who had similar shirts made up for his children, Zilber checked with lawyers to make sure he could use the words and logos.

The lawyers told him "you cannot copyright a word. You can copyright a work, but not a word," he said, citing the use of the word "Camelot" as an example.

Then he did a trademark search. When nothing showed up, Zilber filed for his own trademark. The application is pending at the U.S. Patent Office in Washington.

Warner Bros. lawyers had no comment. But company spokeswoman Monica Bouldin said of the clothing business: "They shouldn't be doing that."

Warner Bros. plans to make a live-action movie based on the Harry Potter books and will probably market its own merchandise when the movie comes out, said Judy Korman, a spokeswoman for Scholastic Books, the U.S. publisher of the series.

Zilber is way ahead of them. He plans to continue selling his own line of Potter merchandise. Customers love it, he said, and are even buying the shirts and hats for grown-ups, something he didn't anticipate.

"If I'm not in violation of a trademark or a copyright, should Warner Bros. be able to say 'stop it' just because they're Warner Bros.?" he asked. "This is America. What can they do to me?"

Then he answers his own question. "They can sue me. . . . If a judge says stop doing it, I'll stop doing it."