The search for Getaround, which helps car owners rent out vehicles and counts Yahoo! Inc. CEO Marissa Mayer among its investors, took more than a year and a shift in business plan. Sam Zaid, the company's CEO, said he contacted between 60 and 80 carriers and, at one point, resorted to cold calling insurance executives he found on LinkedIn Corp.'s website.

Getaround initially planned to offer its service on university campuses. Insuring a pool of students was a snag for some carriers, said Zaid, so the startup shifted its focus to urban areas. The service is available in cities including San Francisco, Austin, Texas, and Portland, Oregon.

An insurance wholesaler eventually put Getaround in contact with Berkshire, which agreed in November 2010 to underwrite coverage for three months, said Zaid. He later secured an annual policy from Buffett's firm for liability, collision and theft during a rental.

"We definitely pay a premium still, given the early nature of the industry, but I think that's OK," said Zaid in a phone interview. "Paying a premium to get a company like Berkshire felt like the right trade-off."

$1 Million Limit

Accidents in recent months have shown what can go wrong as sharing gains popularity. In February, a car that was rented through a service called RelayRides crashed in Boston, killing the driver and injuring four people in another vehicle. The startup has a liability policy with a $1 million limit. That amount could be exhausted, Jonathan Karon, a partner at Boston- based Karon & Dalimonte LLP, who represents one of the injured people, said in a phone interview.

"Almost any other car on the road would have been covered by a fraction of the million dollar coverage available for this claim," Alex Benn, RelayRide's vice president of business development and trust and safety, said in an e-mailed statement. "This tragic incident is an outlier from our many thousands of reservations."

RelayRides, which received funding from the venture-capital arms of Google Inc. and General Motors Co., operates in 46 states and its website highlights cities including Chicago and Los Angeles.

Lloyd's Of London

A San Francisco resident came back from a trip last year to find her apartment vandalized after renting it out on Airbnb Inc. After the incident, the startup partnered with Lloyd's to cover all bookings on the site for as much as $1 million in property damage at residences in countries including the U.S., Germany and Australia. The guarantee doesn't provide liability protection for the host.

"What occurred last summer was a tragic event," Emily Joffrion, a spokeswoman for the company, said in an e-mail. In addition to the $1 million guarantee, the company has responded by hiring staff to promote trust and safety.