If anyone knew the ropes about Airbnb rentals, Daniel Rusteen would seem to qualify. He worked for the company as an accountant and later wrote a book for hosts, “Optimize Your Airbnb.”

A leaner vacation-rental market

SF short-term rentals transformed as Airbnb, others make peace with city. Under the threat of huge penalties, Airbnb, HomeAway, FlipKey and others have jettisoned hosts who ignored the city’s registration requirement

But despite being steeped in all things Airbnb, Rusteen said he didn’t realize that he and his roommates were supposed to register when they rented out a sofa in their North Beach apartment to Airbnb travelers from 2014 to 2016. They used the money for utilities.

“It was all kind of hazy, the whole process,” he said. “We just figured, ‘Okay, we’re just doing a couch. This thing is to keep housing on the market, and we’re not kicking out tenants.’”

The city got wind of the couch rental last year and slapped him with a fine of several hundred dollars, which he considered “ludicrous.” He now travels the world, staying in Airbnbs and running his businesses.

Rusteen’s story isn’t unique. Many hosts claimed ignorance of the registration rule. Others said it was too cumbersome. Others chose to defy it.

San Francisco worked closely with Airbnb on a pioneering vacation-rental law requiring all local hosts to register starting in February 2015. The city wanted to make sure that only permanent residents rent to tourists, so landlords don’t turn vacant apartments into lucrative ad hoc hotels. By getting hosts to register, it can verify that they are permanent residents and require them to report on how often they rent to tourists.

But only a fraction of hosts registered. That left the city playing cat-and-mouse games to track down illegal operators.

In frustration, San Francisco supervisors got tough. They passed a law to levy steep fines and criminal penalties on websites that brokered rentals in unregistered units. Airbnb and HomeAway sued — and essentially lost. A judge ordered them to work with the city on a registration system. Their agreement included a phase-in to jettison unregistered hosts over the months from September until Jan. 16.