Rick Johnson, a private investigator in Denver, recalled two recent cases in which women who were going through divorces hired him because they believed that their husbands were following them. He found GPS trackers on their cars and removed them.

“It scared the hell out of these women,” Mr. Johnson said.

Sales of GPS trackers to private individuals may have already surpassed more than 100,000 per year, some experts believe. The marketing is just getting started.

Danny Burnham, the general manager of InTouch MVC in Lakeland, Fla., said that he was negotiating with Best Buy, Radio Shack and Brookstone and that he hoped to be selling trackers in the big retailers before the end of the year. The devices will be described as safety tools, but no one can be sure of buyers’ intentions.

“Selling a tracking device is similar to selling a firearm: you don’t ask what they are going to use it for, and what they do with it is entirely out of our control,” said Brad Borst, the owner of Rocky Mountain Tracking in Fort Collins, Colo. His company sells GPS devices online, including a 4-inch-by-2.5-inch model called the Ghost Rider, for $349, that can, with a waterproof box and magnet ($30), be hidden under a vehicle.

One Los Angeles man who went through a nasty divorce said he used zip ties to attach a similar device to the car his wife was driving, which was registered in his name. He suspected that his wife, who had said she had health issues and could not work, was giving false testimony.

“I couldn’t eat, I couldn’t sleep, and then I got that thing,” the man said. “It showed the car at the place of business. It showed that she was going out to nice restaurants. It showed she was living a lifestyle above what she had when she split up with me.”