Every tech startup needs a good founding legend -- started in a garage, launched as a way to trade Pez dispensers, funded through the sale of a VW bus and an HP calculator.

But Ian Johnstone would rather he didn't have his. When he was 10 years old, his father was gunned down by muggers in San Francisco. He later died from his injuries.

"To be honest, it's a little hard to remember," Johnstone, 31, says of the 10-year-old boy emotions that the terrible tragedy conjured up. What he does remember is that his mother became very active in efforts to reign in the number of guns on the nation's streets.

"I was always really proud of the work that my mom was doing and I wanted to be a part of it,'' Johnstone says. "There was almost this responsibility to do something about this."

Ian Johnstone,  next to a collection of family snapshots and news articles detailing his father’s murder, talks about his gun buyback website,
Ian Johnstone, next to a collection of family snapshots and news articles detailing his father's murder, talks about his gun buyback website, Gunbygun.com, Wednesday afternoon, Jan. 28, 2014, at his home in San Francisco, Calif. Johnstone's father, David Johnstone, was a visiting book publisher who was fatally shot 21 years ago while visiting San Francisco. (Karl Mondon/Bay Area News Group)

But this is America and the cause is guns; and really you can't do much about guns in America. The politics are lined up against action, as Republicans and Democrats alike cower in the face of the National Rifle Association's money and clout. So, Johnstone, an entrepreneurial sort who lives in San Francisco, and his college buddy, Eric King, came up with a novel approach: A crowd-funded gun buyback enterprise called GunbyGun.

"Ultimately, the political process is broken on this issue," says King, an innovation specialist with the U.S. Agency for International Development, who lives in Washington D.C.

So, why not turn to a market-based solution?

"That is something that we see as being truly an American way forward," King, 31, says. "It's this idea that we're going to allow people to act in their own self-interest to improve the safety in their own communities."

People who have guns and don't want them, sell them. GunbyGun, which wants fewer guns on the streets, pays for them and -- with the help of law enforcement agencies -- destroys them. The outfit, which Johnstone runs full-time, is only a year old. It's been a part of three buybacks in San Francisco, Oakland and San Jose. (So far, GunbyGun has worked with government agencies and community organizations that also put up money for guns.) For its part, GunbyGun has repurchased about 500 guns, Johnstone says, paying $100 for most guns and $200 for assault weapons.

GunbyGun is another sign that crowd-funding has potential beyond launching the next Pebble smart watch or the newest nifty iPad cover. It is fast becoming a force for social change, providing the backing for political campaigns and causes ranging from sending surgeons to save lives in India to feeding the hungry in Silicon Valley.

It's the early days, too soon to know how influential the crowd will be in curing the world's ills. But sometimes, like with gathering unwanted guns, every little bit helps.

"I have no problem with the second amendment," says Santa Clara County Deputy District Attorney Johnny Gogo, citing the constitutional clause that provides a right to bear arms. "At the same time, as a prosecutor, I know there are far too many firearms in our community. A gun buyback has everything to do with people turning in unwanted firearms, unused firearms, neglected firearms, so they're not used to harm, injure or possibly kill someone."

Gogo has helped arrange several gun buybacks in the county, including a San Jose event in December that was partially paid for by GunbyGun. The idea? Fewer guns means fewer shootings. Who could argue with that?

Well, of course some could. This is guns in America, remember? Johnstone and King have heard most of the anti-buyback arguments: The purchased guns only go back into circulation. Most of the turned-in guns don't work, anyway. No criminal is going to turn in a perfectly good gun, even for cash, so you're buying legally registered guns from law-abiding citizens.

To which Johnstone replies: Not true. Also not true. And who cares?

First, the buybacks are run by agencies like the San Jose Police Department, which, with the help of GunbyGun and several government agencies and community groups, bought back 463 guns in December. The guns are inspected to make sure they're in working order before the department pays up. And the guns are destroyed, not recirculated. As for criminals' guns vs. law-abiding citizens' guns?

"Our point of view is that they're all dangerous, and they all pose a threat to society," Johnstone says. "For example, the gun used in my father's shooting, it was stolen from a home just two weeks before he was shot with it."

Obviously, it is far too late to keep that gun off the streets. But the havoc, grief and despair that it created may at long last be fueling a solution that will rid the streets of many more.

Contact Mike Cassidy at mcassidy@mercurynews.com or 408-920-5536. Follow him at Twitter.com/mikecassidy.