On a warm sunny Saturday, we are headed for a family birthday party. We're in Berkeley, the party's in Benicia, we're already late, and so we're warning each other: Not one more delay. But then, crossing Solano Avenue, we see a yard-sale sign. Wordlessly we diverge from our route, arching left to where balloons mark the spot halfway down the block.

It's a big one, stretching from two blue satin cushions propped against a telephone pole all the way up a driveway and across a lawn strewn with clothes, books, tools, kitchenware, toys. As luck would have it, the sale is ending just as we arrive. From her porch, the seller announces that the sale is over and everything is free.

Not all sales end this way. Some folks pack their unsold stuff back up and haul it inside or drive it to Goodwill. Some, though, give it all away. When this happens, the air begins to buzz and everything speeds up. Hands dart. Receptacles are sought to hold the DVDs, the baseball mitt, the vase.

At any free-for-all, you can immediately spot the inexperienced. Gobsmacked and half-embarrassed, they hover over pomanders and clocks but reach for nothing, as if fearing germs or tricks or being called thieves.

And you can spot the scavengers. We glide, our movements purposeful and lithe. Our eyes cut wide arcs, back and forth, as we reach with one hand for shirts — stripes, button-down, okay — and with the other for swim goggles, garden gloves, a blender. Yoink! Into the backpack pops the spoon, the copper horse, the coffeepot. Quick. Competent. Assess each item in a nanosecond. Do I want this? Do I need it? Does my friend?

We will be late to the party. And when we get there and they ask why we were late and we say we were scavenging free clothes and toys and kitchenware from a stranger's lawn, they laugh. They think we're joking. When they see we're not, they scrunch up their noses and ask: But why? Wasn't it dirty? What if someone bled on those shirts? What if someone cooked meth in that coffeepot? What if the blender's broken? Can't you afford gloves?

We hear this all the time.

And: What if that thing doesn't fit?

What if it's dented/scratched/stained/faded/ripped?

Wouldn't you rather just go to a store and buy the exact color/size/style/components you like best?

Well, no.

Because all around the world, a change is afoot. The way in which human beings acquire stuff is shifting. Expanding. Forever. All around the world, millions are salvaging stuff, trading stuff, recycling stuff. This is the end of the shopping monopoly.

All around the world, we are scavenging. Today that doesn't mean only the squalid ragpicking it used to mean. So many pursuits count as scavenging today. They can no longer be tucked into any easy little category. In our new book The Scavengers' Manifesto, my coauthor Kristan Lawson and I define scavenging as any legal means of acquiring stuff that does not involve paying full price. That includes everything from thrift shopping to swapping to bartering to coupon clipping to Freecycling to seeking out the sale racks at Macy's. In the broadest sense, you scavenge just by tracking down a bargain.

But four thousand years of antiscavenger prejudice dies hard.

Pretty much since the dawn of civilization, when hunting and gathering was suddenly seen as primitive, this practice of collecting the lost and discarded, the fallen and unwanted, has been scorned. We see it derided in Leviticus and in modern popular culture, where athletic teams are named after predators and carnivores — and the occasional herbivore, if you count mustangs and bulls. But you never see a sports team named the Hyenas or the Roaches. Scavengers get no respect, even though roaches actually run faster than cheetahs for their sizes and even though all scavengers play crucial roles in ecosystems, working as nature's cleanup crews, and even though our closest evolutionary cousins are omnivores, which means that chimpanzees and Homo sapiens are all scavengers deep down.

But hey. In times of economic tankage, scavengers know what everyone else now needs to learn, like it (and us) or not.

For a long time now, the very idea of getting stuff by any means outside the standard retail channel and at any speed but warp speed was anathema. A sacrilege. A sin. Not long ago, all of American society pledged loyalty to new-and-improved products. Not shopping was treason. An abomination. But times have changed. Goods and services now circle and circle the world, connecting strangers, without a penny spent.

Well, that took long enough.

My coauthor and I have both scavenged for as long as we can remember. We didn't know each other when we were kids, but both of us picked up spare change from the street as soon as we could walk. I grew up near a beach and was frequently taken there. Found shells, smooth pebbles, sea glass, driftwood, sand, and dried seaweed turned into countless dollhouse furnishings and early art projects even before the first day of first grade. He grew up in Berkeley, where friendly hippies at neighborhood communes showed him how to construct furniture from scrap wood and pick plums from sidewalk trees. Ten years before we met, both of us bought metal detectors, in both cases going halfies on the purchase with our best friends, and in both cases choosing the cheapest Radio Shack model. Once we met — both, that night, wearing thrift-shop clothes — neither of us ever had to explain or apologize to the other for bending down to pick up pennies or for lifting trash-can lids to peer inside. We have watched (former) friends look on in horror as we do these things.

Sometimes scavenging is the Great Divide.

Some folks scavenge for fun. Some scavenge to save. Money. The world. While millions all around us drown in debt, we scavengers liberate ourselves with every cent we save while liberating tons of would-be garbage. How can we tell typical consumers that every saved penny counts, that saved pennies add up? They call us cheap. They call us poor. No matter how or why you do it, even if you're just reusing Christmas ribbon or picking fruit in a vacant lot, some will consider you a dangerous radical. Yet the typical consumer carries a four-figure debt. How can we make them understand that they accumulated these debts by paying full price? How can we say: What would you prefer, discount-outlet food and library DVDs and thrift-shop shirts and no debt, or restaurant food, cinema tickets, department-store clothes and debt? Scavengers know that the difference between brand-new, full-price products and their scratched secondhand counterparts is — debt.

Some folks scavenge to recycle. Repurpose. Reduce. Reuse. They know that some 200 million tons of trash is thrown out every year in the United States alone. In New York City, 64,000 tons per week.

Some scavenge to revolt.

Some scavenge to survive.

Some scavenge for the sake of spontaneity. We crave the long-forgotten magic of the random.

Some scavenge for art. Some scavenge for adventure. Some scavenge for self-sufficiency. For some, scavenging is a test. For some, it's spiritual.

We do not all scavenge for the same reasons, yet we share certain understandings, certain values, certain principles. We share a way of life. A way of looking at the world. Having, each of us, shattered the chains that locked us to consumer culture, we walk free under a clear new sky, scanning a changed terrain studded with buried treasure.

We seek but do not always find. This makes each find a miracle.

Amen.

Admit that you have scavenged anything and expect questions. Misconceptions. Misperceptions. Even accusations. One friend tells me that she cannot even imagine donning a garment that someone else has owned before. The very idea, she says, disgusts her. Another friend buys used clothing, no problem, but avoids my favorite discount food store, Grocery Outlet, because "it's like shopping in a warehouse." And if that warehouse stocks organic, fair-trade, and top-brand products just a little closer to their expiration dates than identical merchandise at ordinary supermarkets for half-price or less, what's not to like? Another friend looks at me pityingly and says, "How can you stand making such sacrifices? I would never want to walk past Starbucks and think, I refuse on principle to buy a Frappucino."