When Linden Lazarus founded the vintage watch business Oliver and Clarke in late 2018, he set up operations in a location that made it easy for him to work online late at night: his dormitory bedroom.

After all, he was just 15 and in his sophomore year of high school at a boarding school in Kents Hill, Maine, about 65 miles northeast of Portland.

“It was a little thing I was doing,” Mr. Lazarus said (with a 38-millimeter Patek Philippe yellow gold Aquanaut on his wrist) during a phone interview from the company’s current location, a showroom in Los Angeles. “I never expected that it would be anything like it is today.”

That appointment-only space is inviting, with comfortable seating areas and exposed brick walls, but Mr. Lazarus said that about 90 percent of Oliver and Clarke’s sales came from its website and Instagram account, as well as from a limited selection of watches that it sold on the watch market site Chrono24.

While he won’t divulge his revenue, the 19-year-old said he sold about 440 watches last year, more than double his 2021 total. He has four employees — two full time, two part time — and last year he bought a two-bedroom midcentury house in the Laurel Canyon neighborhood of Los Angeles. “We feel very lucky to have achieved a prominent place in the world of vintage watches,” he said.

The majority of Oliver and Clarke’s watches are from the 1980s or earlier. Rolexes are a specialty, often with unusual dials, and there are timepieces from brands such as Audemars Piguet and Vacheron Constantin, too. The company also sells handmade watch straps produced in collaboration with the Italian maker Jean Paul Menicucci and straps by Molequin, which are made in France.

“I don’t really sell things that are not special in some way,” Mr. Lazarus said, “just because I want people to get a watch and be wowed by it when they open the package.”

The average price of Oliver and Clarke’s timepieces is $40,000, he said, but there are many options for less than $10,000, like a 1967 36-millimeter Rolex Oyster Perpetual in steel recently priced at $4,450. Keeping markups comparatively low, Mr. Lazarus said, is a priority.

“I am of the mind-set that I would much rather sell 10 watches to 10 happy clients that all felt like they got fair deals and come back and buy more,” he said. He contrasted his approach with that of some other dealers, who “are of the mind-set that they’ll sell much less watches, but they’re going to totally rip someone off on the deal — and to me, that’s not an honest way of doing business.”

He said he often stayed in touch with clients by phone, sometimes giving advice on potential purchases from other dealers. Mr. Lazarus’s speaking voice matches his age — there’s still a little more than a year to go before he turns 21, the legal drinking age in California — but he is confident that his horological knowledge trumps his youth.

“If you speak to me for 20 minutes, you’ll probably realize that I know what I’m talking about when it comes to watches,” Mr. Lazarus said.

His clients seem to agree.

“You’re just kind of taken aback by when you ask him a question and he, ad nauseam, answers you about one subtle dial detail, or why this particular insert is more desirable than another one,” said Kevin Smith, a Los Angeles-based record producer who purchased a Red Rolex Submariner and a steel Audemars Piguet Royal Oak from Oliver and Clarke last year.

Nicholas Pelzer, a philanthropic investment manager based in New York, met Mr. Lazarus several years ago. “I was like, ‘This guy is so young, but so knowledgeable about these pieces,’” he said. “It was just an odd, sort of paradoxical thing.” In late January, he purchased a 40-millimeter white gold Rolex Daytona from Mr. Lazarus.

Mr. Lazarus grew up mostly in Stamford, Conn., although he spent a few years in Los Angeles as a child. His father, Bruce Lazarus, is a theatrical producer and lawyer; his mother, Laura Lazarus, is a retired social worker and his brother, Revan, 21, is a student.

A family connection inspired Mr. Lazarus when it came to naming his company: He chose Oliver, the name of a deceased family dog, and paired it with Clarke, simply because he liked the combination.

Mr. Lazarus first became passionate about watches in July 2018, during a vacation to London with his father. Instead of enjoying the tourist sites they had planned to visit, the teenager found himself entranced with the timepieces at a large branch of the retailer Watches of Switzerland.

“After that trip I knew that I wanted to do something in watches,” he said, “whether that was to buy a watch or sell a watch or just even own something that was nice, because before that I think I probably had a $50 Seiko, but nothing that was a real nice true watch.”

Not long afterward, he gathered his savings — mostly payments for doing odd jobs, but also the proceeds of collectible baseball card sales that he began at 9 — and bought a 40-millimeter steel and rose gold IWC Ingenieur on eBay for $2,800. A few months later, he sold it for an $800 profit.

“I was so over the moon and loved the process,” he said. “I thought I’d made this money and it was so much fun. From there, I wanted to get started.”

He began educating himself about watches, often watching YouTube videos. And he began making connections in the watch world, attending collectors’ gatherings and hanging around the jewelry district in New York City. (His parents frequently dropped him off since he was too young for a driver’s license.)

Mr. Pelzer, who met Mr. Lazarus during that period, said, “What I appreciated about him was that he was young, but he was also really hungry.”

By the time he was a high school junior, Mr. Lazarus decided to skip college because he felt he “would make more than an entry-level job out of college just doing the watches.” He graduated from high school in 2020 and moved to Los Angeles the following year.

In some ways, Mr. Lazarus’ youth might be an advantage — his dexterity with online sales, for example, seems generational.

“Obviously the younger people are, the better they are at it,” said Rory McEvoy, executive director of the National Association of Watch & Clock Collectors, referring to selling watches online. (Mr. Lazarus is not an active member of the organization.)

Eventually, however, the market will change again, Mr. McEvoy said. “The next generation will come and probably run rings around them with the technology that’s available.”