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Lisa Harrington felt unfulfilled with her corporate life and in need of a side project.

“I really didn’t like my job. It wasn’t going anywhere. So, I ended up, for fun, just selling stuff on eBay from my closet — old purses, shoes, anything I could get my hands on,” she told Business Insider.

As the items in her closet dwindled, she figured the next logical step would be creating her own product to sell online.

“I just fell into this e-commerce research hole — this whole world of being able to analyze supply and demand,” she said. “Eventually, I found a dog niche.”

She decided to sell dog harnesses, found a manufacturer through Alibaba, and ordered about $5,000 worth of inventory using her eBay earnings. The product arrived at her doorstep, and “it took me three hours to get them upstairs because it was carton after carton after carton,” she said.

Harrington listed and sold most of her initial inventory on eBay.

This was in 2012, when selling on Amazon was still uncharted territory. But when she discovered the relatively new platform and created a listing, “they sold out within 24 hours,” she said. “And so I did the math: If I actually kept this in stock, how much would I do a year in revenue? And it was easily a million dollars.”

Her instant success on Amazon likely had more to do with timing than her product.

“In 2013, 2014, there were not many options out there, so it was easy,” she said. “Every year gets harder.”

From dog harnesses to cat doors

For years, Harrington sold dog harnesses while working her 9-to-5. She ran a lean operation from her home in Arlington, Massachusetts, using the internet as a business coach: “Google was my best friend. I taught myself how to import, how to find a factory, how to wire payment, how to do marketing, how to find a bookkeeper, how to read a profit and loss statement.”

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By 2016, her e-commerce profit matched her salary, and she left the unsatisfying job that encouraged her to start a side project in the first place.

Having gained an extra eight hours a day, she launched her second brand, Purrfect Portal, to fill a gap in the pet space.

Harrington has designed and patented multiple interior cat doors. Courtesy of Lisa Harrington

“I am a cat lady — I’ve always been a cat lady — and I really wanted an interior cat door,” said Harrington. “When I went online, there was only one option.”

And it wasn’t great, she added: “It was the quintessential, ‘I can do this better.’ I know how to do this. I’ve found a factory in China. I’ve figured out how to bring cargo from one part of the world to another.”

Still, designing a cat door would be a completely different and more complex project than producing dog harnesses. A mentor she found through the business program SCORE connected her with engineers and a manufacturer who would help create the plastic product.

Her cat doors were profitable but not an immediate smash hit.

“It was the smaller of my two businesses, and it was just chugging along and growing year by year,” said Harrington, who started to notice a recurring customer comment: People were desperate for a closable door, rather than a cat door with flaps.

The solution to her customer’s woes was born out of a happy hour with friends.

“Someone said, ‘What about a miniature door? A little human door with a little doorknob and a little window and a little door knocker,'” recalled Harrington. “And everyone just lost their minds, even the non-cat people. And in my gut, I knew people would love it.”

It took two years to bring the happy hour vision to life, but when it launched in 2020, “people just lost their minds,” she said. “It catapulted to the top of the bestseller list pretty quickly. We were selling 100 units a day.”

In 2025, “we’re the No. 1 selling cat door on Amazon, and we have awesome patent protection on the invention,” she added. BI confirmed her seven-figure revenue by reviewing her Amazon sales dashboard.

Keys to succeeding on Amazon: a reliable manufacturer, a network to lean on, and an excellent product

Making money on Amazon in 2025 is harder than it’s ever been, said Harrington, who offered three pieces of advice to prospective sellers.

First, if you don’t find a reliable manufacturer, you could fail before you even list your product.

“They can make or break your business in terms of really producing something that’s high quality, and then also the trust around wiring money to the other side of the world,” said Harrington.

After nearly a decade of working with the same factory, she’s making a major adjustment in response to the unpredictable tariffs. By October, “80% of our catalog is going to be made in the USA, which was not on my BINGO card,” she said.

The switch isn’t to cut costs. In fact, producing in the States will cost her more. But she said the peace of mind is worth it: “I’ve honestly just had so many sleepless nights over the tariffs. I’ve been doing this for 10 years. I’ve never been in a scenario where my cost of goods could double overnight or triple overnight, and I just couldn’t handle that stress anymore.”

Harrington and her team of three employees work remotely. Courtesy of Lisa Harrington

The next ingredient in the success recipe is your network. You want to surround yourself with people who have already done what you’re aiming to do.

“I can trace so many aha moments and big successful moments in my business to conversations with certain people who are more experienced or further down the road than I am,” said Harrington, who is a member of an elite group of seven-figure Amazon sellers called Million Dollar Sellers.

A major perk of the membership is access to events, where Harrington can connect with peers in person.

“You end up leaving with pages of notes and to-dos,” she said. “There’s always one conversation — it could be five minutes or an hourlong — but there’s always some conversation you have with someone randomly that just completely transforms what you’re going to work on when you get home.”

Finally, to make money on Amazon in 2025, you need to offer an excellent and unique product.

“In some cases, you’re competing with brands that are the factory or have a lower cost of goods, so it’s really hard as an American brand to compete that way,” she said. “I think to really succeed on Amazon today, you need to have a moat — and the moat is either something that is sourced in the United States, something that is very difficult to make, or intellectual property like a utility patent.”

Once you select your product, “make it as amazing as you possibly can, set a low price, spend on ads, and give it time.”