BI has spoken with entrepreneurs who make money buying and reselling items on sites like eBay.Top resellers tend to focus on a specific category, like clothing, sneakers, or electronics.They use data to inform their purchases and stay up-to-date on market trends. Thanks for signing up! Access your favorite topics in a personalized feed while you're on the go. Business Insider has spoken with a handful of individuals who have had so much success reselling items as a side hustle that they quit their day jobs to do it full-time.One particularly successful reseller, Richard S. who prefers not to share his last name for privacy reasons, is planning to retire before 40, thanks to the eBay store he started curating after losing his job in 2008.While the startup costs are low another full-time clothing reseller, Kendall VanGilder, told BI how she got started with no money upfront and anyone can get involved, there's a strategy behind sourcing profitable products."You can't just list garbage, for lack of a better term, and expect a great result," said Richard S.He, VanGilder, and Val Zapata, who has sold millions in sneakers and streetwear via live shows on Whatnot, shared their top sourcing strategies. BI verified each individual's revenue claims by looking at screenshots of their seller dashboards.1. Niche downRather than selling a variety of items, each successful reseller BI has spoken with sells within a specific category and it tends to be a category they're already familiar with.VanGilder has worked in retail most of her career, including spending five years at Lululemon, and uses that to her advantage. Reselling clothing "was a natural move for me because I have the ability to be able to pick up on brands that are good just by fabric or feel of it," said the owner of Style Secondhand.Zapata leaned into sneakers, as she'd been collecting them with her dad since she was a teen. She now has a 6,000-square-foot warehouse to house all of her inventory. Val Zapata turned her sneaker-collecting hobby into a seven-figure business. Courtesy of Val Zapata Richard S. has found it helpful to niche down within his niche.His general strategy is to buy "the staples," brands like Nike, Lululemon, and Ralph Lauren that people wear daily. Within those big brands, however, he's learned that certain items do better than others.For example, he figured out that while a regular Nike Dri-FIT shirt might sell for $10 to $20, one with a Roger Federer or Rafael Nadal logo might go for $30 to $70, or even up to $100 if it's the same style one of them wore in a recent tennis tournament.With Ralph Lauren items, he's looking for something specific, like a polo with a big pony logo and "Germany" spelled out on the front, he said. 2. Lean on data and pay attention to market trendsTake the guessing out of what will sell and figure out exactly what customers want using the data available to you.Richard S. advises looking at what is selling well by browsing the "sold items" section of your category on eBay for 30 minutes a day.Sourcing items based on data will help you avoid the common mistake of buying what you personally like and assuming other people will as well. In the reselling business, "it doesn't matter what I think is cool," he said. "It matters what the customer thinks is cool." Richard S. inside his vintage clothing store, The Spot, in Coral Springs, Florida. Courtesy of Richard S. VanGilder uses eBay to learn about new brands in her space. For example, she searches a broad apparel category, like "women's pants," and filters by items that have already sold for $50 and above. Browsing the results helps her discover higher-end brands that she'll then try to hunt down the next time she's shopping."There have been so many times when I've done research on a brand that I'd never heard of, and then I will go to a thrift store and find it," she said. "It was probably there all along, but now that I'm bringing it into focus in my mind, I'm seeing it."Zapata says she stays up-to-date on what's popular in the shoe world by attending events, paying attention to what people are wearing, and observing what her favorite sellers are buying and avoiding.3. Consistency winsThe combination of assessing the market, experimenting with various listings, and general experience will set you up to find "home run items," said Richard S. "If you go out there with the most amount of information and you give yourself the most amount of opportunities you will run into more of these home run items."He's not seeking these rare items on a day-to-day basis, he noted: "When people hear these conversations, they think you just magically find all these $100 or $500 or $1,000 items. You do find them. But the bulk of the items that you find are going to be more of the everyday value and common items that are in high demand. They are out there, but you have to touch a lot of items and you have to give yourself a lot of opportunity to be able to find them."