Antonella Carbonaro, a consultant to financial technology companies, saved up to buy her Birkin bag, a luxury tote made by Hermès that sells new for tens of thousands of dollars. Since getting her bag in 2018, Ms. Carbonaro has stored it in her closet, bringing it out only on special occasions.

But when she heard that there was a marketplace to buy shares in other Birkins, including more exotic versions that can fetch six figures, she was in. It is not a lark. Ms. Carbonaro, 30, sees her shares in an exclusive bag as an alternative investment, no different than stakes in private equity funds that invest in a basket of companies.

“This is a visual way to participate in different asset classes that aren’t as accessible,” Ms. Carbonaro said. “Investing in shares of Birkin bags, even though I have one, is getting more exposure.”

She bought 10 shares in a Bleu Lézard Birkin bag that was valued at $61,500 in an offering last year. Earlier this year, she bought 25 shares in a gray Himalaya Birkin. It was valued at $140,000 in an offering in May.

Unlike owning a fractional share of a condominium, she will never be able to use her investment. Shares are traded until the owner of the marketplace sells the asset.

Ms. Carbonaro’s first Birkin investment is trading up 6 percent from the purchase price on Rally Rd., a platform that deals in fractional investments in collectible items. The other one is still in the lockup period and its shares cannot be traded yet.

The market for investing in fractions of items otherwise seen as collectibles — and largely reserved for the wealthiest people — has seen an uptick in interest during the pandemic as people spend more time at home.

Rally Rd. began by selling shares in exotic cars several years ago but has expanded to art, books, wine and whiskey, memorabilia and Birkin bags.

“In the beginning, it was like equity markets: just safe, blue-chip investments,” said Rob Petrozzo, a founder and the chief product officer at Rally Rd. “Over the past few months, we’ve seen with people being inside, they’ve gotten access to more information and they have been exploring the app more fully.”

He said existing investors on the platform had doubled the number of items they owned shares in. Initial offerings have sold out five times faster than before the pandemic, as new investors on the platform began buying up shares more quickly.

To accommodate growing interest, MyRacehorse, which sells shares in racehorses that are far smaller stakes than those sold by traditional racing syndicates, has partnered with a top stud farm, Spendthrift, to extend the length of the investments. Before, its model had been to sell the horse when it was done racing. Now, investors can participate in the breeding fees, which can be many times any racetrack winnings.

The fractional movement is not limited to luxury items. Fidelity, the mutual fund giant, offers “stocks by the slice” where you can buy a portion of a share starting at $1. And many private equity funds, which have high minimum investments and long lockup-periods, have created mutual fund versions of their funds.

Eugene Olmstead, a retired internet technology executive, said he had 1 percent to 1.5 percent in 11 horses, all bought through his self-directed individual retirement account.

“You’re not going to get a worthwhile return on your investment unless you have a certain percentage,” said Mr. Olmstead, 58. “I’ve done my research, and I’m investing in ones that I think in the long run will give me a decent return.”

Of the 11 horses he has bought shares in, only two are old enough to race. He said both had average winnings of $12,000 a race. He has received some dividends from those races, but said the money was not substantial yet.

“It’s money I don’t need right now,” he said. “It gives me a chance to wait for those returns.”

Another owner of fractional shares in horses, David Falo, 58, compared buying stakes in young horses to investing in companies on private platforms before their initial public offering. “The horse may not do well, or the horse could get injured,” he said, “but it gives you a little thrill along the way.”

There are many caveats. Trading through Rally Rd. and MyRacehorse are done through apps, which makes buying and selling easier and creates a community. But the apps turn investing into games, as has happened with the stock-trading app Robinhood. That can distort the financial consequences of ill-considered investments.

Compounding the risk, an asset typically bought for personal enjoyment or bragging rights cannot be analyzed the same way that a private equity investment would be.

“There could be return potential, but who knows?” said Jack Ablin, chief investment officer of Cresset Capital. “There’s no liquidity and no control. When do you get your money back? You don’t know. The other is the carrying costs could be high.”

In the case of the shares in the racehorses, expenses like training and boarding are shared just as profits are. “You own full equity in the horse,” said Michael Behrens, founder of MyRacehorse.

Another issue is that buying these assets in slices can mean a person is paying more than she or he might if the person could buy the whole asset, and that could dampen returns or make it hard to resell the asset.

“You’re buying an overvalued slice of the whole,” said David Abate, senior wealth adviser with Strategic Wealth Partners. “If you decide you want to get out of this investment, you’d better understand how the secondary market works.”

The fees are disclosed but baked in. With MyRacehorse, 15 percent of the offering of a horse goes to the company upfront. But each horse is part of an entity that has been registered with the Securities and Exchange Commission.

“This is high risk; I’d never tell people otherwise,” Mr. Behrens said. “We’re not trying to build a platform that says this is going to be a really good asset class. Many horses have been bought for $1 million and never made it to the racetrack.”

As with other alternative investments, buyers are restricted from the selling of these fractions until after the lockup period ends. But when the asset itself — the bag or the horse — is sold is determined by the platform, not the individual investors.

Jimmy Lee, chief executive of the Wealth Consulting Group, a wealth adviser, questions the notion of buying a passion asset with an eye toward profit. “When it comes to art, you only see the ones that go up in value,” he said. “If someone buys a piece of art for $1 million and it doesn’t go up in value, it’s not going to be sold.”

There are other drawbacks. These marketplaces do offer the possibility of a return on the investment, but they deprive people of the joy of owning a painting or a fast car: having it in your possession. (Although with MyRacehorse, investors can at least go to the track and see their horses.)

“You lose the intimacy of what it’s meant to be,” Mr. Ablin said. “It’s normally an asset you can touch, enjoy, ride in, ride on or drink.”

But many investors in shares seem unbothered by this. Ms. Carbonaro said not being able to touch or hold the bags she had invested in was not an issue for her. “If I had a Michael Jordan rookie card, I don’t think I’d want to touch it,” she said.

John Cochran, who works in sales in Baltimore, has invested in shares of 76 different collectibles including a shirt Mr. Jordan wore in a basketball game, a Muhammad Ali fight contract, a portrait of Abraham Lincoln and a 2006 Ferrari f430 manual.

He said he was happy receiving a photo and some information on the object and was unfazed that he could not hold or touch it. “I like the idea that, just like my stocks, it’s all in an electronic portfolio,” he said. “I don’t have to have the resources to store these things.”