Twenty years ago, Bertrand Brillois, a Parisian businessman, began contacting seamstresses, costume designers, fabric dyers, production assistants and others who had worked for Prince. He told them that he thought Prince was not only a musical genius but also a fashion icon, and he wanted to buy clothing, jewelry and other accessories designed or worn by him.

The many items acquired by Mr. Brillois over the years included an ankle-length white cashmere coat that Prince had custom-made by a tailor in Nice, France, when he was filming the 1986 movie “Under the Cherry Moon.” The coat, along with more than 200 other items, is on sale as part of the Fashion of Prince, an online auction that is accepting bids through Nov. 16.

The sale, held by RR Auction, also features one of Prince’s signature wardrobe items: a white, high-necked, silk shirt with elaborate ruffles, puffy sleeves and faux pearl buttons. Prince wore it, according to the auction company, when he performed a blistering rendition of “Purple Rain” during the American Music Awards ceremony at the Shrine Auditorium in Los Angeles on Jan. 28, 1985.

The collection goes beyond outfits worn by the artist who was sometimes known as His Purpleness, including backstage Polaroid shots, notes handwritten by Prince and master tapes of the albums “Lovesexy,” “Batman” and “Diamonds and Pearls.”

There are also concept sketches and a binder containing fabric swatches in various shades of purple that offers clues on how Prince and his wardrobe team created his singular style and image.

“You can see the creative process by which Prince and these designers were making these garments,” said Bobby Livingston, an executive vice president at RR Auction.

Mr. Livingston mentioned as an example the yellow lace suit with an exposing backside that Prince (in)famously wore to the 1991 MTV Video Music Awards. “The butt suit — there’s fabric from that garment,” Mr. Livingston said. (The cheek-baring ensemble was later revealed to have panels that covered Prince’s bottom. The ensemble itself is not part of the sale.)

At a preview party on Tuesday night at the Chelsea Hotel in Manhattan, the displays and the accompanying catalog provided intimate glimpses of the auction’s subject. Prince’s hat size was 7⅛. The high heels of his custom boots — there are four pairs up for auction — were reinforced with hidden metal brackets, to prevent them from breaking during his exuberant stage shows.

Tinu Naija, an editor of Shoeholics magazine, had come with the celebrity stylist Phillip Bloch. “Prince was one of the original influencers,” she said. “There’s got to be some shoes to check out, some jewels to gawk at.”

Mr. Bloch eyed Prince’s gold cuff links that spelled “Sexy” and said he hoped Santa Claus would bring them for Christmas. “He was all about accessories,” Mr. Bloch said.

Santa may need deep pockets. The auction market for Prince has boomed since he died in April 2016.

A few months after his death, the Hollywood auctioneer Profiles in History sold a ruffled shirt and a blazer worn by Prince in the film “Purple Rain” for $96,000 apiece, well above the asking price of $6,000 to $8,000. In 2017, Julien’s Auctions sold one of Prince’s custom-made “Cloud” guitars for $700,000, far surpassing the $60,000 to $80,000 estimate.

In 2020, RR Auction sold a Yamaha DX7 synthesizer played by Prince for nearly $74,000 — three times the expected price. In June, the auction house sold the demo tape that won Prince his first recording contract, for more than $67,000.

The singer’s estate, which was initially left in disarray after Prince died without a will, is not affiliated with RR Auction or the current sale. Mr. Livingston said Prince was known to give things to employees and friends, adding that he held garage sales at Paisley Park, his production studio and headquarters in Chanhassen, Minn.

Mr. Brillois, the French collector, flew in from Paris to attend the Chelsea Hotel party and bid adieu to the collection he had spent years assembling. He never had any contact with Prince himself and said that former employees of Prince thought he was crazy for wanting to buy stuff they had stored in closets or considered throwing away. But as a Prince fan, he saw the value — not as a speculator but as a preservationist.

“For me, I was thinking it has to be preserved,” Mr. Brillois said, adding that he consulted experts at the Louvre Museum and the Musée des Arts Décoratifs about how to set up a climate-controlled environment to store the vintage garments, jewels and paperwork.

Mr. Brillois said that at one point he had hoped to one day open a museum to Prince’s fashion. But after Paisley Park was turned into a museum by the singer’s estate, he felt that Prince’s legacy was in safe hands and decided to part with his collection, which, though impressive, is far smaller than what is displayed in Minnesota.

Mingling with guests at the party, telling stories behind this or that item, Mr. Brillois was in a happy mood. “My work is done,” he said.