“Your happy envelope is on its way!” This was the title of an email that appeared not long after I’d sent two dead laptops and two dead cellphones to Gazelle, an online gadget reseller. More thrilling was the $397 check that followed. It was a happy ending that sent me burrowing into my closets, hoping to find more expired booty to sell.

It’s been 21 years since the founding of Craigslist and eBay, both of which transformed so-called “person-to-person trading.” In the last decade, companies like Gazelle have refined that process again by removing the messy human factor. With sleekly designed websites, mailers and labels for free shipping, e-resellers handle the sales of your goods themselves, often paying you up front. As the life spans of your things grow ever shorter, and you are increasingly overwhelmed, to paraphrase a Kate Atkinson character, by “the relentless culling and resolution that the material world demands,” selling your stuff without having to leave home would seem to be an enchanting innovation and do much to dull the sting of an object’s obsolescence.

Clothing resellers like Material Wrld, Crossroads and thredUP propose to make “refreshing” your wardrobe more joyful, with their own trade-in kits and cash incentives to shop their wares to keep the cycle going. Ethical elimination is a theme (a corollary to ethical consumption). The manifesto of Crossroads, a favorite of college students who worry that their Urban Outfitter discards may end up in a landfill, is that “fashion shouldn’t come at a cost.” Material Wrld aims to alleviate “fashion guilt” with its own promise: “We’ll handle yesterday’s fashion so you can focus on tomorrow.”

As fashion gets faster, these services are multiplying, flush with venture capital.

“It’s the Age of Consignment,” a friend proclaimed, still giddy from selling off the contents of her basement.

Tradesy is like a dating site for your old clothes: You can post a photo, tell its story and the site will price your garment (a button invites online shoppers to “love” your listings). Move Loot will do the same for your furniture; if a piece sells, the company will handle the exchange and arrange for pickup. So will Lofty, Chairish and Viyet, which sell high-end furniture, decorative items and artwork; curators from Lofty and Viyet will vet your items in your home. The luxury site the RealReal, a favorite of fashion-conscious New Yorkers, trades in artwork, designer clothing and jewelry.

Material Wrld operates out of Industry City, a groovy former factory building in Sunset Park, Brooklyn, now colonized by tenants like West Elm’s Makers Studio and the Storefront for Art and Architecture. Its founders, Rie Yano, 34, and Jie Zheng, 33, met at Harvard Business School and left their jobs in fashion to start the company in 2012.

Like many fashion resellers, they make you an offer upfront, and send that which they won’t accept to a charity, in their case Housing Works. “What we’re encouraging is a lifestyle where if you purchase quality fashion,” Ms. Yano said, “we can extend its life when you don’t want to wear it anymore.” In late March, Material Wrld received $9 million in financing from an e-commerce company based in Japan.

“Resale is trending,” said Martin Ambrose, 29, an associate manager at the RealReal, who estimates he assesses over 2,000 items each month from clients, many of whom are regulars. The company, in business since 2011 and with $123 million in financing, reports that it takes in, on average, 100,000 items each month and has sold about two million items. “People used to be embarrassed,” Mr. Ambrose said. “But once you start, it’s like an addiction.”

Mr. Ambrose, who wrote a college essay about the characters he met while window shopping, is a resale encyclopedia. Some basics: Women’s pants don’t sell. Nor do men’s dress shirts, because people are very particular about these items. Recognizable brands, like Christian Louboutin shoes with their red soles, do. Shoes and handbags hold their value nicely. The RealReal divides the sale price 40/60; once a seller’s wares reach $10,000, that person receives 70 percent. In the world of luxury consignment, Mr. Ambrose said, that benchmark is not the unicorn you’d imagine.

I met him one chilly February afternoon at the home of Emilie Cresp, 33, a natural-beauty entrepreneur who had decided to cull her formidable wardrobe, of which over 350 pieces (by Chanel, A.P.C. and Vanessa Bruno, mostly) were being stored at Garde Robe, the high-end closet and valet service, for up to $2,000 a month.

I had realized pretty quickly that my Gazelle sale was a fluke. I owned nothing else of value — at least, nothing that I wasn’t using — and if I wanted to participate in the Age of Consignment in a meaningful way, I’d have to do so as an observer. (“Get out of my room, Mom,” my daughter had texted from college after I’d sent her photos of her own left-behind clothing. “Do you really want this?” I’d written. “Need stuff to sell.”)

Ms. Cresp, who is French, lives in a small one-bedroom apartment in the West Village with two full closets and a stuffed armoire. I had been invited there by Ann Lightfoot, 55, a professional organizer in Manhattan with a wide network of consignors and charities who sends next to nothing to the Dumpster. Who knew there was a booming market for used Lululemon workout gear? Ms. Lightfoot’s company, Done & Done Home, which she runs with her daughter, Kate Pawlowski, 29, cleans closets at an extremely high level (chief executives, film and TV directors, an Olympic athlete and a “Real Housewife” have been clients).

She told me that many of them were inspired by Marie Kondo, the Japanese tidiness guru, but were flummoxed by the process. “They don’t know where to begin, and they worry about where those things that don’t spark joy will end up,” she said.

It took six hours to sort through Ms. Cresp’s clothing, a typical marathon, Ms. Pawlowski said. She and her mother leaven their labor with brisk good humor and a refreshing lack of dogma. “We could color-code,” Ms. Lightfoot said while folding all 70 of Ms. Cresp’s sweaters. “But that can stress out a client.” By sunset, Mr. Ambrose had carted away four suitcase-size zippered totes; Ms. Pawlowski left 11 garbage bags at Goodwill, and sent four bags to Linda’s Stuff, an online consignor based in Pennsylvania.

A few days later, Ms. Cresp mused on the process in an email. She had decided, she said, to pull some Vanessa Bruno items back from the RealReal to give to friends because the prices weren’t as high as she’d hoped and it made more sense, she wrote, to offer those things to people she loved. As Mr. Ambrose had noted, the more well-known brands received the highest prices: A Chanel turtleneck dress, for example, was marked $895. Of the 90 or so items Mr. Ambrose removed, 10 were rejected for sale.

“It’s always very sad when things come back,” said Judith Thurman, the author and a New Yorker writer whose stirring fashion pieces let you know that she is both a critic and a passionate collector (for those of us reared on the writings of Kennedy Fraser, Ms. Thurman’s work has long been a habit). Ms. Thurman consigns the old-fashioned way, among a series of shops in her Yorkville neighborhood that she visits regularly, she said, like a lobsterman checking her pots, “to see what the tide has washed in.”

“I have really good stuff, but like everyone, I make mistakes, I get carried away,” she said. Nodding to Ms. Kondo, as one must, she added: “When you do interrogate your objects, even if they don’t spark joy, they’re telling you something. Why do I buy endless numbers of white raincoats? I always have to give them away because they make me look like a dental hygienist. But it’s a tic I have.”

Joan Juliet Buck, the author, actress and former editor of French Vogue, said: “Everyone’s objects are freighted. It’s never just stuff.” In January, Ms. Buck purged her 42nd Street loft and sold some belongings through Paddle8, the online auctioneer: Art Deco lamps, a Cartier watch, an Hermès desk set, vintage R. Crumb comics and lots of jewelry, including a long strand of steely gray South Sea pearls that she’d bought a few years into the job at French Vogue. “I was miscast as an executive,” she said. “The pearls were austere, very expensive, and I thought they telegraphed authority. I sold my mother’s jewelry to buy them.” And yet, she reported sadly: “The boss lady pearls didn’t sell. They’re coming back.”

This is why consignment can still break your heart. Not every object you send away has a happy ending.

Decades ago, when a magazine I worked for went out of business, my father very kindly bought me a suit to wear on job interviews. It was double-breasted, by Claude Montana, and God help me it was also taupe. Is taupe, I should say, because I still have it. I wore it once, to an interview with Carrie Donovan, who was the style editor of The New York Times Magazine, and despite its taupeness, she hired me. “Pantsuit,” is what my new colleagues called me, and who could blame them? I never wore the thing again, but I saved it — it was a designer outfit, after all, and cost more than my rent at the time — carting it through many moves and many closet purges.

Two months ago, flush from my Gazelle success, I listed it on Tradesy, noting its provenance: “Worn once, to a job interview in 1990 (I got the job!)” Last week, the listing expired, having garnered zero “loves” on the site, and no happy envelope.