At the third Duane Reade of the night, Anna Sacks, 31, a dumpster diver who goes by @trashwalker on TikTok, hit the jackpot. Half a dozen clear trash bags sat along Second Avenue not far from her home on Manhattan’s Upper East Side.

Kneeling on the ground, Ms. Sacks untied the bags with a gloved hand and, using her iPhone flashlight, pulled out her haul: Tresemmé hair spray. Rimmel London Stay Glossy lip gloss. Two bags of Ghirardelli sea salt caramels. Six bags of Cretors popcorn mix. Wet mop refills. A Febreze air freshener. Toe warmers. A bottle of Motrin. All of it unopened, in the packaging and far from the expiration date.

“Oh my God,” said Ms. Sacks, digging out a 6-pack with one can missing. “My mom loves Diet Dr Pepper.”

The total value was perhaps $75, but money wasn’t the point. Ms. Sacks, a former investment bank analyst, films her “trash walks,” as she calls them, and posts the videos to expose what she sees as the wastefulness of retailers who toss out returned, damaged or otherwise unwanted items instead of repurposing them.

Fed up with the profligate practice, dumpster divers like Ms. Sacks have started posting videos of their haul on TikTok in recent years as a way of shaming corporations and raising awareness of the wasteful behavior.

A search of #dumpsterdiving on TikTok brings up tens of thousands of videos that collectively have billions of views. They include a video by Tiffany Butler, known as Dumpster Diving Mama, who found several handbags in the trash last year outside a Coach store in Dallas, all of them apparently slashed by employees. Ms. Sacks bought the bags and made a TikTok calling out the fashion brand. After the video went viral and sparked outrage (and was picked up by Diet Prada), Coach said it would stop “destroying in-store returns of damaged, defective, worn and otherwise unsalable goods,” and instead try to reuse them.

Most of the dumpster activists target mass retailers like CVS, TJ Maxx, HomeGoods and Party City. Luxury fashion brands tend to keep a tighter control over their excess inventory and sometimes pay to have unsold items burned.

A video posted this month by Liz Wilson, 37, a mother of two in Bucks County, Pa., who goes by Salty Stella, shows a dumpster at a nearby HomeGoods store filled with Halloween-themed mugs, plates, dog bowls and holiday decorations. “This is absolutely horrendous,” Ms. Wilson told her 1.2 million TikTok followers. “The only reason these things were thrown away is because Halloween is over.”

Ella Rose, who goes by GlamourDDive, posted a video two months ago showing a dumpster outside a TJ Maxx store, filled with Zara dresses, grooming products by Fekkai and clothing from Victoria’s Secret.

At a time when corporations tout their commitment to the environment, the sight of $500 handbags or even $6 Ghirardelli chocolates discarded in a dumpster can be a bad look.

“Corporations don’t want people to see the overproduction, the wastefulness, the lack of donation,” said Ms. Sacks, who has 400,000 followers and has received significant media coverage. “To change behavior, it’s important to expose the wastefulness.”

Michael O’Heaney, executive director of The Story of Stuff Project, an environmental group in Berkeley, Calif., that raises awareness about waste through storytelling, called Ms. Sacks and other eco-minded dumpster divers “metal detectors for flaws in the system.” “What they’re finding in the trash are a fascinating lens into our waste economy,” said Mr. O’Heaney, whose organization recently filmed a trash walk with Ms. Sacks.

Some do more than just raise awareness. Ms. Wilson puts together “Stella’s Kits” — which contain feminine hygiene supplies like pads, tampons and flushable wipes assembled from dumpster dives — and distributes them at homeless shelters and other places where women experience what is known as period poverty.

While Ms. Wilson also posts to YouTube and Instagram, she said that her videos get the most reactions on TikTok. “People are just shocked and saddened,” she said. “Every day, I get the same reaction: ‘Oh, my god. Why do stores do this?’”

Mark Cohen, the director of retail studies at Columbia Business School, said that the practice is based on the cold calculation that “the simplest and most expediate way for a retailer to dispose of something, typically of low value, is to mark it out of its stock and dump it.”

Merchandise that was returned cannot always be resold because of regulations meant to protect consumer’s health — including food, some over-the-counter drugs and health and beauty aids, Mr. Cohen said. Items that have been damaged or worn, or are out of season like holiday decorations, may have lost too much value, even for third-party buyers.

“As egregious as it is to see seemingly perfect product put into a landfill,” Mr. Cohen said, “it’s the shortest and least expensive path.”

Activists like Ms. Wilson and Ms. Sacks would prefer to see retailers donate items to charitable organizations and others in need. “We should be incentivizing corporations ideally to produce less in general,” Ms. Sacks said, but if that’s not possible, they should “donate or sell it through, or store it for the next year, rather than destroy it.”

Many retailers say that they do, in fact, donate unsold goods, but some merchandise still needs to be sent to landfills. “The thought that everything leftover can be donated is a nice thought to hold,” but unrealistic, Mr. Cohen said.

CVS, for example, said it diverted 50 percent of its unsold merchandise last year to recycling or reuse, and donated about $140 million worth of goods to charities including Feeding America. CVS works “with nonprofit organizations to arrange for damaged or near-expired goods from our stores to be donated to communities in need,” said Ethan Slavin, a spokesman.

Andrew Mastrangelo, a spokesman for TJX, the parent company of TJ Maxx and HomeGoods, said that “only a very small percentage of merchandise from our stores goes unsold,” and that most of the unsold merchandise is bought by third parties or donated to charities.

Walgreens, which owns Duane Reade, said it donated 10 million pounds of goods in 2021. “Walgreens works diligently to divert from landfill unsold or discontinued products such as food, toiletries and household items,” said Candace Johnson, a spokeswoman.

Even so, some items cannot be donated, including perishable products within one month of expiration. “Products that do not meet applicable standards for donation or liquidation,” Ms. Johnson added, “may be discarded in the trash.”

Discarded merchandise is perhaps most abundant around the holiday season. Last Halloween, Ms. Wilson said she found more than 120 Halloween-themed dish towels outside two HomeGoods stores near her home, all in perfect condition.

Ms. Wilson has a circuit of dozens of retailers around southeastern Pennsylvania that she visits every week. She never comes up empty. “I could go to a dumpster today and get a bunch of stuff,” Ms. Wilson said, “and go back to the same dumpster 24 hours later and find new stuff in it.”