Concerned about robberies, scams and killings tied to purchases arranged through Craigslist and other online marketplaces, police and sheriff’s departments nationwide are carving out areas of their stations for people to complete transactions.

At least 70 departments in the U.S., from Boca Raton, Fla., to Bedford, Texas, have created such zones, usually in parking lots or lobbies, according to websites that track the programs. Though the trend began several years ago, it has taken off in recent months, authorities say.

...