Thredup's report highlights the waste in the fashion food chain.

Paula Sutter, a board member of Thredup and former president of Diane von Furstenberg, extols the virtues of buying vintage in the online marketplace’s fourth annual resale report. “It’s consumption without the elements that make consumers feel guilty or ‘conspicuous,'” she says in a letter. “The secondhand industry is gaining incredible momentum. With heightened interest from consumers, investors and retailers, online resale is becoming a way of life.”

Sutter sounds positively evangelical when she adds that “online resale is changing the rules…It’s not a question of if, but when hundreds of millions of people all over the world make resale their new healthy habit.”

Resale is a growth industry, according to Thredup, which projects that apparel resale by 2025 will be a $25 billion market. That’s an 82 percent increase from 2013. By contrast, e-commerce sales of apparel will grow by 14 percent and retail overall will advance 3 percent in the same period — although both from much bigger bases.

By some estimates, over $8 billion worth of clothing in American closets is unused and ignored. Thredup believes that the success of online resale will help unlock a large portion of this “hidden treasure.”

The growth in online resale is being fueled by wallet capture from adjacent markets such as value retail and offline thrift. Value retail, which includes off-price retailers, outlet stores, discounters and department stores, is a $175 billion market and growing. Offline thrift is a $12 billion market with an increasing number of consumers migrating online in search of a wider selection of brands and styles, which will hamper meaningful growth for offline thrift in the coming decade, the report said.

At the same time, the online resale market has been consolidating. Thredup, The Real Real, Poshmark, Tradesy, Vestiaire Collective and Vinted are all that’s left from a field of 14.

The report is full of facts such as: If one in 100 American households shopped resale, they would collectively save $1.4 billion a year. If one in 100 American households shopped resale, 99 billion gallons of water would be saved. And if one in 100 Americans cleaned out their closets, 140 million found items could be resold.

Thredup’s prescription for consumers is “reduce, reuse, recycle.” The company believes that resale is healthy for the full-price retail sector because when consumers clean out their closets, they make room for new items.

As the velocity of closet turnovers increases, consumers get more use out of the clothing in the ecosystem and fewer items wind up in landfills. About 85 percent of all donated clothing ends up in landfills. Only 15 percent of donated clothing is recycled, with most ending up overseas.

On a more personal level, Thredup says that 93 items in a typical closet, or 70 percent, are never worn. That comes out to $8,500.

Thredup says retailers and resellers need to work hand in hand and the report highlights efforts by retailers such as H&M and Madewell, among others, to reduce waste.

Mobile is driving a large portion of resale growth with 68 percent of resale activity occurring on mobile.

Brands with the strongest resale value, in descending order, are Theory, Seven for all Mankind, Diane von Furstenberg, Tory Burch, Joe’s Jeans, Kate Spade New York, Eileen Fisher, J Brand, Vince and Citizens of Humanity. In terms of mainstream brands, Ann Taylor, J. Crew, Banana Republic, Gap, H&M, Express, The Limited, Nike, BCBGMax Azria and Talbots made the top 10 list.

Value brands were led by Old Navy, Forever 21, Mossimo, Merona, Ann Taylor (Factory Outlet), Gap Outlet, H&M and Label Of Graded Goods, Style & Co., L.L. Bean and Arden B.

The top 10 most active brands at resale are J. Crew, Forever 21, Free People, Mossimo, H&M, Nike, White House | Black Market, Michael Michael Kors, Lucky Brand and Ann Taylor. Breakout resale brands include Lululemon Athletica, Talbots, Athleta, Joe’s Jeans, Kate Spade New York, Eileen Fisher, Coach, Theory and Under Armour.

Thredup lists the most active resale cities as New York, Washington, D.C., Los Angeles, San Francisco, Boston, Philadelphia, Chicago, Seattle, Atlanta and Houston.