Lauren Eager got into thrifting in high school. It was a way to find cheap, interesting clothes while not contributing to the wastefulness of fast fashion.

In 2015, in her first year of college, she downloaded the app for Poshmark, a kind of Instagram-meets-eBay resale platform. Soon, she was selling as well as buying clothes.

This was the golden age of online reselling. In addition to Poshmark, companies like ThredUp and Depop had sprung up, giving a second life to old clothes. In 2016, Facebook debuted Marketplace. Even Goodwill got into the action, starting a snazzy website.

The platforms tapped into two consumer trends: buying stuff online and the never-gets-old delight of snagging a gently used item for a fraction of the original cost. During the Covid-19 pandemic, as people cleaned out their closets, enthusiasm for reselling intensified. It was so strong that Poshmark decided to go public. On the day of its initial public offering in January 2021, the company’s market value peaked at $7.4 billion, roughly the same as PVH’s, the company that owns Calvin Klein and Tommy Hilfiger, at the time.

Then, the business of old clothes started to fray.

Using the Poshmark app, Ms. Eager and others said, started to feel like trying to find something in a messy closet. The app was cluttered with features that did not work or that she did not use, and it felt “spammy,” she said, sending too many push notifications.

Many platforms found selling used items hard to scale. Now, online resellers are trying to recalibrate. Last year, ThredUp decided to exit Europe and focus on selling in the United States. Trove, a company that helps brands like Canada Goose and Steve Madden resell their goods, purchased a competitor, Recurate. The RealReal, a luxury consignor, appointed a new chief executive as the company tried to improve profitability.

Poshmark is undergoing perhaps the biggest reinvention. In 2023, Naver, South Korea’s biggest search engine as well as an online marketplace, bought the company in a deal valued at $1.6 billion, less than half its IPO price.

Something of a mash-up of Google and Amazon, Naver is betting it can rebuild Poshmark, which has 130 million active users, with the same technology that made Naver dominant in its own country.

It may also help breathe new life into the resale market. Analysts think the resale fashion market still has room to grow in the United States, with revenue expected to increase 26 percent to $36.3 billion by 2028, according to the retail consultancy firm Coresight Research.

New legislation in California could help. The law, passed last year, requires brands and retailers that operate in the state and generate at least $1 million to set up a “producer responsibility organization” to collect and then reuse, repair or recycle its products. Resale platforms like ThredUp and Poshmark could be in a position to help brands carry out that mandate.

At the moment, though, Naver’s focus for Poshmark is more basic: Make it a better place to sell and shop. The company has the “operating know-how” to do that, said Philip Lee, a founder of the media outlet The Pickool, which covers both South Korean and U.S. tech companies.

“They’re trying to renovate Poshmark and then expand the market share,” he said.

A Marriage of Search and Commerce

Poshmark, which is based in Redwood City, Calif., was founded in 2011 by Manish Chandra, an entrepreneur and former tech executive, and three others. In trying to expand, Poshmark faced a problem common to resellers: Capturing the excitement of the secondhand-shopping treasure hunt while not frustrating buyers with an endless scroll. The company knew it needed better search, as well as interactive elements that gave people more reasons to come beyond paying $19 for a J. Crew sweater.

For its part, Naver was looking for ways to push beyond South Korea, where its commerce and search businesses were already mature. The growing online resale market in the United States presented an opportunity, and also gave the company access to the largest consumer market in the world.

“Commerce is a big growth engine for us,” Namsun Kim, Naver’s chief financial officer, said. And the peer-to-peer sector, where users sell to one another, was still in its infancy, with room to expand. But, Mr. Kim added, “it’s a more challenging segment, and that’s why it’s harder for a lot of the larger players to enter.”

There are two common business models for resale: peer-to-peer and consignment. With consignment, a platform collects and redistributes physical goods. Poshmark uses the peer-to-peer model, which relies on scores of people — many of them novices — haggling over prices and then mailing items to one another. This decentralization can be a headache for brands, which like to maintain a certain level of control of their products. And platforms like Poshmark must make buyers comfortable with trusting the sellers on their site.

Before the Naver purchase, it was difficult to push through needed technological changes, said Vanessa Wong, the vice president of product at Poshmark.

“I would always talk to my engineers and ask, ‘What if we do this or do that?’ They’re like, ‘That’s hard. The effort’s really high,’” Ms. Wong said.

Naver’s purchase offered both the investment and the expertise to pull off the changes. Founded in 1999, the company is everywhere in South Korea.

“We are not just a simple search technology or A.I. service,” said Soo-yeon Choi, the chief executive of Naver, whose headquarters are near Seoul. The company, she said, “alleviates the frustrations of people, which is what is needed to help growth.”

Search built Naver “into the massive power that they are in Korea,” said Mr. Chandra, who stayed on as chief executive after Naver’s purchase. It was the top priority when the company bought Poshmark.

Several new elements for users and sellers have been introduced. With a tool called Posh Lens, users can take a photo of an item and, using Naver’s machine-learning technology, the site populates listings that are the same or similar to the shoe or tank top that they’re searching for. A paid ad feature for sellers called “Promoted Closet,” pushes listings higher on customer feeds.

Poshmark also introduced live shows, some of which are themed, to draw in the TikTok generation and increase engagement. One party auctioned off clothing previously worn by South Korean celebrities, a connection that was made with the help of Naver.

Still, the resale market is going through growing pains and has not quite found its footing since the height of the pandemic. It’s not clear whether the changes taking place at Poshmark will be enough. In May, Mr. Kim, Naver’s finance chief, said in an earnings call that Poshmark’s profitability was improving, but by November, the company was cautioning that growth had slowed because of weakness in the peer-to-peer resale market in North America.

Missteps and Reinvention

The company has already done some backpedaling on unpopular decisions.

In October, Poshmark introduced a new fee structure, which increased costs for buyers. Sellers, fearing that higher costs would make consumers bolt, revolted. Within weeks, the company scrapped the new fee structure.

And there are still user headaches: tags and keywords that help users find what they’re looking for can be miscategorized. Sellers sometimes tag their products incorrectly to get more eyeballs on their less popular products. (Hard-to-offload Amazon leggings, for example, may be listed as Free People apparel.)

The company is beta testing changes with its frequent sellers — people like Alex Mahl, who sells thousands of dollars in apparel on the site each year. And within dedicated Facebook groups related to Poshmark, there’s a lot of chatter about the changes that sellers and buyers would still like to see.

“The only way for it to do well is there’s going to be constant changes,” Ms. Mahl said about the tweaks on Poshmark. “If you were just on an app that never changed — one, it would be boring, and two, the opportunity to just do better wouldn’t be there.”

One recent morning, Ms. Eager, the seller who joined Poshmark back in college, was pleasantly surprised to find that the app had some new features she actually liked. She snapped a photo of her Aerie gray tank top with Posh Lens. Within seconds, the app populated listings of similar products. It was so much better than conjuring up the adjectives needed to describe it.

“Love it,” Ms. Eager exclaimed.