Carl Pardiny, chief of police of Fairfax City, Va., stands in the newly established “Exchange Zone” outside police headquarters, where citizens can complete online transactions. (Tom Jackman/The Washington Post)

The rise of Craigslist, and the easy online sale of unwanted computers or concert tickets, has been accompanied by a dark side — the tense moments when strangers must meet to complete the deal.

The stories are plentiful: a string of armed robberies in Prince George’s County in June of people selling phones; a college student slain in Missouri in May after agreeing to sell a car on Craigslist; a job seeker in Ohio killed and buried by a farmer.

But a new idea is emerging with local police departments around the country to combat the problem: A “safe exchange zone,” clearly marked in the parking lot of a well-lit police station, or even in the lobby of the police station, for people to meet and sell their goods with less fear of a quick or violent rip-off.

The practice appears to have started last year in Boca Raton, Fla., spread through central Florida and then through the law enforcement grapevine to police parking lots and lobbies throughout the country. Leesburg and Fairfax City in Northern Virginia have adopted the approach in recent months, as a way to give online buyers and sellers a sense of security when they meet.

“Over the summer of 2014,” said Boca Raton Officer Sandra Boonenberg, “we had three or four different robberies where the victim had made arrangements to meet someone to see either an iPhone or a computer. They met them in public places — one happened at a gas station — and they still got robbed. We decided we’re going to have to come up with something better, and the chief [Daniel C. Alexander] came up with the idea to use the police department for transactions.”

Officers do not get involved or actively monitor the transaction, although many police parking lots and lobbies have surveillance cameras as an added backup. In some smaller departments, such as Fairview Heights, Ill., east of St. Louis, an officer might go outside and meet the buyer and seller just to make public contact and reassure them, Lt. Mike Hoguet said.

“I had concerns about the unintended consequences” of online dealmaking, said Fairfax City Chief Carl Pardiny. He said Fairfax had incidents in which robbers had gone to a seller’s residence, snatched an electronic item “and they’re gone.” The chief also said he felt uncomfortable as he accompanied his daughter to sell concert tickets to a stranger at a shopping mall.

“I’ll gladly give up two of my parking spaces,” Pardiny said, “to create an atmosphere that we’re at a location that’s safe. Policing is not about law enforcement completely. It’s about providing quality-of-life-based services to our residents and business owners.”

Fairfax City marked its safe exchange zone in September, but Leesburg was the first in this area in August. “It was starting to be a trend nationwide,” Lt. Jeff Dube said. “We looked at it and we’ve gotten a lot of good feedback. With more and more people shopping online, it gives them a sense of security.”

Police parking lots have been used quietly for years to make child-custody exchanges between divorced parents. But now the police are publicly inviting online sellers and buyers to use their space.

Fairfax County is considering trying the exchange zones formally, but some residents have begun using district station lobbies or parking lots anyway, Lt. Brooke Wright said. The idea has not yet been tested in Maryland or District departments contacted by The Washington Post.

Although the number of Craigslist and other online transactions conducted safely is in the millions, the meet-ups that go wrong have attracted plenty of attention. One online blog tallied 45 homicides related to online deals between 2009 and 2014, including Philip Markoff in Boston, known as “the Craigslist killer.”

The exchange zones are too new in Leesburg and Fairfax City to claim any success, but they are being used elsewhere. Hoguet said that recently in Fairview Heights, he sold something on Craigslist and the buyer told him, “I’ll only meet you at the Fairview Heights Police Department.” Hoguet replied, “Okay, I’ll walk outside.”