One of the nation’s most popular smartphone apps is increasingly becoming a ticket to trouble.

Since the start of 2017, the OfferUp app — which allows people to sell items to each other without a retail store or a middleman — has led to the arrests of at least five people on robbery charges in Palm Beach County and the Treasure Coast. With the click of a button to open an app and a few messages to choose a meeting spot, would-be robbers, all between the ages of 16 and 25, have tried to turn people equipped with cash to buy or a phone to sell into easy victims.

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OfferUp stresses it does not "tolerate criminal activity." It bills itself as a safe alternative to websites such as Craigslist that have long dogged by security problems, down to the point of sending police agencies signs to designate safe-exchange zones in their lobbies. It allows people anonymously to post a picture of an item to sell and chat with potential buyers. The exchanges mostly happen without harm, but some have had to hand over items while staring at a gun.

Last week, a 19-year-old West Palm Beach man arrested on a charge of armed robbery after using the app to set up a meeting at a Greenacres community clubhouse to buy a pair of Nikes, the Palm Beach County Sheriff’s Office said. The teen pulled out a gun instead of cash to get the shoes.

His case isn’t rare. Police departments are noticing the trend and putting out guidelines for buyers and sellers.

The Boynton Beach Police Department invited the public in January to use its lobby, open around the clock, as a safe spot to make person-to-person transactions, and people have been using it since, said spokeswoman Stephanie Slater. Already this year, Boynton Beach police arrested two 16-year-olds and charged them with stealing a man’s cellphone when they met through the app. The department also responded to a report of a woman who said she received counterfeit bills during an OfferUp transaction, Slater said.

"It’s likely to be a legitimate transaction if they will meet you in the lobby of a police department," said Slater, who noted that items a would-be robber can grab — such as cellphones — are especially susceptible to thefts through these apps.

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As of Tuesday morning, OfferUp was the top grossing shopping app in the iTunes App Store. Following close behind is letgo, which is a similar app that has been used in similar crimes, authorities say. The Florida Department of Law Enforcement doesn’t keep statistics specifically of robberies that stem from these apps, but Special Agent Donald Cannon said common sense often gets forgotten when people see how easy they are to use.

"If it’s too good to be true, it’s not," he said.

The robberies with these apps are similar to when Craigslist became popular, Cannon said. But as the website’s robberies began dwindling with public knowledge of how to prevent Craigslist crimes, marketplace-app robberies have been rising with those apps’ popularity.

Cannon advises to offer only a minimum amount of personal information when using these apps — just a first name should be fine, he said. These apps already use users’ approximate geographical location to connect with local buyers and sellers.

"I’m sure the developers of the app never thought those types of things would occur," he said.

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The app — known for its slogan of "Buy. Sell. Simple." — also offers its own guidelines on how to stay safe, such as looking at the person’s profile to see reviews and to meet in a well-lit public location.The app recommends communicating only inside the app because it can review those conversations and to contact the Trust and Safety team through the app if issues arise. It also includes a list of places it has designated as safe to make exchanges.

"The trust and the security of our users is a core focus for us at OfferUp and we do not tolerate criminal activity," the company said in a statement. "We have worked closely with local law enforcement in Palm Beach County."