Last June, my mother Rita retired after working the same job for 31 years. She’s 60 and energetic (she takes a hip hop dance class), so she wanted to find a way to fill her time, but was strongly against getting another job.

In the past, my mom had casually bought and sold things on eBay (EBAY). Now that she’s retired, reselling used items online has basically become her part-time job. She spends her mornings shopping at local thrift stores and her afternoons photographing and listing those items online. The house has gotten a little cluttered with her inventory, but she’s happy. She’s on a retirement budget, but the eBay sales serve as a supplement – she pulls in a not-too-shabby $600 a month or so that she can use to travel or spoil her grandchildren.

By now we know that eBay is an online marketplace where people can buy and sell anything from shoes to TSA body scanners, but before my mother’s reselling obsession, I was unaware of just how profitable it could be.

According to eBay, retail growth with eBay Established Commercial Sellers (small-business sellers) saw 20% annual growth between 2010 and 2014. And in the first quarter of 2017, the eBay marketplace (where sellers list their products) earned $1.18 billion in revenue. Independent online sellers, in many ways, are reaping the benefits of the overall slump in department store sales and steady increase in online retail sales; a look at this table from the Federal Reserve Bank of St. Louis tells the story. The use of technology has also allowed this community of small-business owners to resell their goods internationally, opening them up to a literal world of opportunity.

To get a better idea what it takes to make real money in this space, we talked to three entrepreneurs who resell full time. From what to buy to where to sell it, they share their experiences in this booming era of e-commerce.

Becoming your own boss

For Josh McAlary, 26 , becoming a full-time reseller was about having the freedom to choose his own fate.

As a competitive runner who was recruited to run track and cross country in college, McAlary was never one to back down from a challenge. That is, until he decided to get his MBA from Michigan State University. “The program was great, but they were teaching me how to work for another person, and that was not what I wanted to do,” McAlary told Yahoo Finance. “I realized that I wanted to be an entrepreneur.”

McAlary focused his efforts on eBay reselling, a hobby he developed during undergrad to pay for expenses not covered by scholarships. In December 2015, McAlary sold $30,000 in men’s clothing, making a $15,000 profit in a single month. Sales are always higher in the fourth quarter anyway (because of holiday gift shopping), but McAlary felt confident that he was on to something.

A few weeks later, he officially dropped out of his second semester of grad school and opened his own LLC, Vogue Squared. He’s carved out a niche in selling men’s apparel, and made $299,000 in gross sales online in 2016. McAlary took home $144,000 in profit. Not bad for a grad school dropout.

To earn such an impressive salary, McAlary says he sometimes puts in 15-hour days. At 7 a.m. he goes to the post office and ships out items that sold the previous day. Then, he’ll head to the thrift stores or discount stores to source for new inventory. After that, he goes home to measure, photograph and list all the new items.

“I can kick back and watch TV while I list and I think that’s relaxing.” he said. “I also have the freedom to take 14 days off for a trip, and still make money by setting my store on vacation mode. There aren’t a lot of jobs that would offer you that luxury.”

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