The founder of Sports Collectors Daily, Rich Mueller, had just returned from the National Sports Collectors Convention at Chicago when he learned a former Miami Heat security officer was facing federal charges of transporting and transferring stolen sports memorabilia.

A former Miami policeman, Marcos Thomas Perez, 62, was arrested earlier this month, accused of stealing millions of dollars’ worth of Miami Heat game-worn jerseys and other items that he later sold via online marketplaces while working for the Heat between 2016 and 2021.

Mr. Mueller wasn’t shocked by the news, saying theft is an ongoing problem in the industry. According to research from Market Decipher, the memorabilia and trading card market will grow to $271 billion by 2034 from $33.6 billion in 2024.

“Anytime there’s an opportunity for someone to steal whether it’s [trading] cards or game-worn jerseys, you’re always going to have an element that looks for that,” Mr. Mueller tells The New York Sun. “They know there’s a market for game-worn memorabilia, especially when you’ve got players like LeBron James and Dwyane Wade and some of those great teams they had in Miami. Anytime there’s big money involved, there’s always crooks doing something.”

The global sports memorabilia industry began to thrive during the pandemic and continues to surge. An example of that growing consumer interest is the more than 125,000 people from throughout the world who attended the five-day Collectibles Convention. “There’s big money involved,” Mr. Mueller said. “There were booths selling 10-cent and 25-cent cards and the next booth was offering a 1952 Mickey Mantle Topps card for $100,000. It’s a big spectrum.”

According to the charging document, Mr. Perez, a 25-year veteran of the Miami Police Department, sold 100 items over a three-year period, including a game-worn LeBron James Miami Heat NBA Finals jersey for an estimated $100,000. The same jersey was later sold at a Sotheby’s auction for $3.7 million.

Sotheby’s is a partner with NBA Auctions, where the league provides memorabilia for sale. The first Dallas Mavericks jersey worn by Cooper Flagg during a Summer League game was sold on Tuesday through Sotheby’s/NBA Auctions for $95,250. It was the most ever paid for a Summer League jersey, surpassing the $62,000 someone paid for Victor Wembanyama’s rookie debut jersey in 2023.

“A lot of teams are more cognizant of what these things are worth,” Mr. Mueller says. “Players are now wearing multiple jerseys during the course of a game, especially if it’s a milestone game. Even if it’s a run-of-the-mill game, someone will tag it and authenticate it, and somebody will buy it if it’s a big name” for $9,000 or $10,000.

Mr. Perez allegedly used his credibility as a police officer to earn the Heats’ trust and access to a secured equipment room that housed hundreds of game-worn jerseys and other memorabilia. The organization was saving the items for a future Miami Heat Museum. A search of Mr. Perez home uncovered 300 stolen game-worn jerseys and other memorabilia.

Miami Police quickly distanced themselves from Mr. Perez. “While he separated from the Miami Police in 2016 and has not been affiliated with our agency for nearly a decade, any betrayal of the public’s trust, past or present, is a stain on the badge and the oath we all take to serve with integrity and honor,” the police chief, Manny Morales, posted on X.

Theft of sports memorabilia has been around for decades, often in brazen fashion. In 2014, men broke into the Yogi Berra Museum and Learning Center at Little Falls, New Jersey, and swiped nine of Berra’s 10 World Series rings. The thieves, who were eventually caught, also took two of his Most Valuable Player plaques and seven other championship rings. One of the thieves was linked to a 2015 break-in at the International Boxing Hall of Fame at Canastota, New York.

In 2017, game jerseys worn by Tom Brady during his last two Super Bowls and the helmet Von Miller wore in Super Bowl 50 turned up in Mexico. An investigation partnering the NFL, the FBI and the New England Patriots disclosed a member of the international media as the culprit.

In 2012, a long-time clubhouse manager for the New York Mets, Charlie Samuels, admitted to stealing more than $2 million worth of memorabilia from the team. The items included autographed jerseys, bats, and baseball. Mr. Samuels had been with the club for 34 years.

“There’s more stringent safeguards in place now because teams are starting to know a little bit more about the fact that these things are valuable,” Mr. Mueller said. “For a long time, the teams didn’t care. Now they understand there’s a significant amount of money to be made.”