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.”