The Brides Wore ’90s Dior. And ’00s Margiela. And ’80s Chanel.

One of the most intimate questions you can ask eBay power users is to show you their negotiations — the private DMs they exchange with sellers bargaining over the price of an item. But Alexis Novak humored me.

The item was a spring 2018 dress from the French luxury brand Céline, then under the creative direction of Phoebe Philo, a designer whose cult following is almost unparalleled in modern fashion. The gown, with long sleeves and a turtleneck and glazed in white sequins, was listed at $8,000. Ms. Novak offered $6,000. The seller politely declined.

Ms. Novak, 32, didn’t push back. She knew her limits. As the founder of Tab Vintage in Los Angeles, she is among the generation of archival fashion collectors helping to change the way celebrities dress. Which means she is experienced in sourcing pieces from auctions, estate sales, resale websites and other dealers — pieces like the Thierry Mugler dresses worn last year by Kourtney Kardashian Barker to the Oscars, and Dua Lipa to her Ibiza birthday party.

But the Céline dress was different. Ms. Novak didn’t intend to add it to her archive for someone else to wear on a red carpet. She collects Ms. Philo’s work, and from the moment she had seen the white gown in runway photos in late 2017, she had thought: “That is my dream wedding dress. That is the chicest, coolest dress I’ve ever seen.”

Five years later — and now engaged to James Valentine, the guitarist for Maroon 5 — that eBay listing was only the second time Ms. Novak had come across the dress on the resale market. Still, she knew her price, and she walked away.

“But I thought about it every day,” she said. “Should I, should I, should I?”

Then, after two months of silence, the eBay seller unexpectedly agreed to Ms. Novak’s price. She bought the dress that day.

Ms. Novak’s wedding looks, from left to right: a 1920s dress for the elopement, a 1970s dress by Halston for the party, and the Céline gown. Credit…Jes Workman

The truth was that Ms. Novak had long found weddings “kind of cheugy,” she said. But since founding Tab in 2019, Ms. Novak, a former comedy writer, had increasingly fielded requests from clients and friends to help them hunt down archival bridal wear — pieces with “character,” she said. “Wearing vintage on your wedding day, you’re not just plucking it off a runway or a rack and wearing what someone told you to wear. You’re infusing your own personality. It’s an extension of you.”

She knew of one brick-and-mortar boutique with this kind of inventory (Happy Isles in Los Angeles) but none with a strong presence online. “I felt like, from a business standpoint, I was leaving money on the table,” she said.

So on Monday, she introduced a collection ofabout85 bridal and wedding guest pieces on the Tab Vintage website, furthering a push into the market that began earlier this year.

In January, she was a co-host in Los Angeles of a three-day bridal and evening-wear event with Re-See, a resale company in Paris. All 72 shopping appointments were reserved weeks in advance. “It was madness,” Ms. Novak said. A tearful altercation between two brides who wanted the same ruched strapless Loris Azzaro dress reminded Ms. Novak of a sign she once saw outside Amarcord Vintage Fashion in New York: “Nothing haunts you like the vintage you didn’t buy.”

Then in February, she teamed up with the online luxury retailer Moda Operandi on a selection of wedding dresses, suits and separates, including a 2004 Balenciaga “puffball” gown (reissued from an original 1951 design) and a 2006 Alexander McQueen silk dress with a gold chain bodice. Both sold for around $20,000.

That sale “didn’t feel so bridal with a capital B,” said Jamie Lenore McKillop, a brand consultant who bought a Martin Margiela couture dress from 2006, made from upcycled knits and silk. It was important to Ms. McKillop, 30, who lives in Los Angeles and is getting married in Sea Ranch, Calif., next summer, to find something singular.

Tab Vintage’s wedding collection includes these gowns by Yves Saint Laurent, Halston and Thierry Mugler. Credit…Allegra Messina

“I’m not the girl who is embarrassed if she goes to a party and she sees someone else wearing the same thing,” she said. “But with social media, certain weddings and dresses will go viral. And then it really becomes her dress — or people associate it with someone else — versus fully your own.”

She’s still looking for her actual wedding dress, however; the Margiela is for her rehearsal dinner. Because that’s the thing about many modern weddings: They aren’t just weddings. They are weekends full of welcome parties, after-parties, bridal-brunches-on-the-beach and aprés-ski-farewell lunches.

Chloe Green-Vamos’s nuptial wardrobe includes a 1970s ivory Yves Saint Laurent suit (“very Bianca Jagger,” she said), which she found in Paris, where the wedding will be held next spring. At the Tab Vintage and Re-See event in January, she bought two Balenciaga minidresses, circa 2000s — one in textured brocade for the wedding brunch and the other in satin (“kind of Mia Farrow, Sharon Tate, ’60s vibe”) for the after-party.

She also acquired from Ms. Novak a silk cream column gown from Christian Dior’s fall 1999 collection, designed by John Galliano, for her rehearsal dinner. And she is “95 percent sure” that her ceremony dress, currently on hold with Tab, will be a peachy-pink chiffon couture gown, fastened with a velvet black bow and camellia, that Claudia Schiffer wore the first time she walked the runway for Chanel in 1989. It is valued at more than $20,000.

Ms. Green-Vamos, 32, who is based in Los Angeles and works in strategy at Estee Lauder, likened her shopping experience to a “fashion Odyssey,” in pursuit of pieces she felt would reflect her personality and story. (She studied in Paris and once worked as a buyer for Chanel, for example.) “I wanted that timeless elegance that these designers have been able to retain,” she said. “This ability to stay modern, despite everything around us changing.”

Ms. Novak herself also wore multiple looks to celebrate her wedding to Mr. Valentine on, yes, Valentine’s Day. After the Céline gown, which had been tailored for a tight fit — its turtleneck was temporarily detached for the occasion — she changed into a more comfortable red jersey one-shoulder Halston dress from the 1970s. (In searching for her perfect slinky Halston, Ms. Novak acquired at least six; some have made their way into Tab’s collection and onto the bodies of celebrities like Madelyn Cline at a Vanity Fair party.) Her veil was an upcycled pink embroidered Dries Van Noten cardigan, found on Vestiaire Collective.

There was also the elopement dress she wore on Feb. 5, when she legally married Mr. Valentine at their home in Los Angeles. The cream gown dated to the 1920s. She had her tailor turn its long sheer sleeves into a halter neckline to make it look “undone but still classic,” she said.

Like some of the best vintage pieces, it had no designer label.