Sharon Watterson sat by the exit doors Sunday at the Great Maine Bike Swap, ready to spring into action as a volunteer security staffer.

She was there to confront anybody trying to abscond with one of the hundreds of bicycles on sale at the annual event, held at the University of Southern Maine’s Sullivan Recreation and Fitness Center in Portland. Watterson said she was the right person for the job, which grew out of a few unfortunate incidents last year when people left with a bike without paying. The job requires both tact and chutzpah.

“I’m a people person but I am not afraid to handle this. I am very comfortable,” she said.

Watterson was one of dozens of volunteers who showed up to help at the swap, where hundreds of used bikes change hands. Organized by the Bicycle Coalition of Maine, the swap takes place in an atmosphere of controlled chaos as shoppers descend en masse to find bargains. Volunteers get first dibs on the bikes the night before the swap in return for their help.

In its 16th year, the swap draws about 2,000 buyers from across the state. The coalition holds a similar swap in Bangor.

The coalition offers pricing guidance to sellers and receives a 15 percent commission on the bike sales. The coalition uses its proceeds to pay for programs.

The swap grosses about $110,000. The amount raised Sunday was not immediately available.

Rod Geant, a volunteer who helped shoppers find the right bike, said his advice to most shoppers was simple: Stay away from the tandems, which he dubs “divorce machines,” and go for the hybrids if you are a baby boomer.

Geant said he was able to find a Trek hybrid for $105, which he will rent out through his Fun and Sun Rentals business in Scarborough. A new one sells for more than $400, he said. In addition to getting a good deal on a bike, volunteering was a way to pay the coalition back for promoting bicycling and facilitating new friendships, Geant said.

“I have been impressed by how nice the volunteers are,” he said.

Martha Palmer of Cape Elizabeth, a longtime coalition member, created her volunteer job, which is to carry a big sign that reads “end of the line.”

She said people used to get upset and unruly because no one knew in the chaos where the line to pay with cash or credit ended. So she came up with the signs and tries to provide some comic relief to help defuse any tensions.

“I feel like a prophet. I tell people, ‘It really is the end of the line for you.’ I make them sing ‘A Bicycle Built for Two.’ The kids love it,” Palmer said.

Volunteer Bill Hall of Peaks Island said helping out at the cash registers was probably the hardest volunteer post at the swap.

“It is the most stressful table. You have to add correctly and subtract,” he said.