Glyn Williams was working as a morning-show host and sales director at a radio station in Derbyshire, England, when he found out he had vocal-cord cancer.The 51-year-old had spent 25 years selling everything you could think of, includingradio advertising and industrial chemicals, but the cancer forced him out of the face-to-face sales game altogether. It derailed his career, and it was hard for him to find a foothold in another business.He left his job in early 2013 and started writing e-books. He churned out 22 titles that still bring him about $1,000 a month.But it was tough. Williams was still struggling after a year of this, and he was beginning to get despondent.Then a bit of luck sneakedinto his life. A fellow author told him about Teespring, a new site whereyou could create custom T-shirts and sell them without having to keep any inventory. It requiresno money down.This seems like a salesperson's dream, butWilliams was skeptical. His friend, however, showed him a shirt hehad made aimed at "country girls" thathad made $600 in one week."Then I sat up and took notice," Williams tells Business Insider.Williams has since become one of Teespring's most successful sellers, recordingover $2 million in apparel sales in 2015 alone.But he's not the only one. Teespring CTO Evan Stites-Clayton says more than20 sellers have made millions on his platform.To understand how Williamsdid it, you first have to understand Teespring.How Teespring worksHere's the basic idea.First, you log on to the site and create a T-shirt design. You can either upload an image file or use Teespring's built-in tools. Then you decide how much you want to sell each shirt for and, by extension, how much profit you will get per shirt. Teespring handles the manufacturing and shipping and takes a cut of the sales.Each T-shirt campaign works a bit like Kickstarter. People preorder your shirt, and when the campaign ends, Teespring ships them outif you have reached a specifiedminimum number. When the campaign closes, Teespring transfers you the money you made. Simple.Williams' first few attempts at a Teespring campaign flopped. One of his early designs featured the words "Stop smoking andvape instead," a nod to the then-emerging trend of e-cigarettes. The sales' Zero.But Williams hit his stride quickly, partly because on Teespring there's no real downside to experimenting until you get it right. Since there is no money down, who cares whetheryour first campaigns don't take flight' All you are wasting is time.Just a month after whiffing on campaign after campaign, Williams cleared $35,000 in net profit. "I started at just the right time and hit on a trend that went ballistic," he says.The big secret: FacebookThere's a secret to making boatloads of money on Teespring that all the big sellers know: Facebook marketing.Marc Boulos, another major seller, who has made over $175,000 in profit in the past year, says you need to pay at least $10 in targeted Facebook advertising for every T-shirt campaign you launch. That, he says, is the magic number at whichyou can start to see whether your campaign has the potential to go viral.The top Teespring sellers are as attuned to the concept of virality as YouTube stars or digital media giants like BuzzFeed (which reportedly spends millions on social-media advertising). Sellers like Williams and Boulos invest in Facebook's ability to get a T-shirt to a place at whichit can spread across the network if it hits a cultural vein.By the end of February last year, Williams was launching 20 shirts per day and working 16-hour days. He was a content machine, eventually having six computer screens running at once.He pumped out 489 separate designs for St. Patrick's Day. One of his tactics was targeting different Irish last names with personalizedcampaigns, such as one for "Gallagher."Targetinglast nameswas a common method used by early Teespring pioneers, though the effectiveness of it has diminished over time, as copycat sellers have crashed into the market. The art of making a Teespring hit usually isn't about any particular cleverness, but rather about finding a specific market and then throwing wave after wave of designs at it.Target: veteransAfter Boulos failed on his first 50 campaigns, losing $500, he got his break by marketing to veterans and their families."I made a shirt that said, 'Some people call me a US military veteranI prefer to be called a retired badass.' It was red, white, and blue,and very patriotic," Boulos says. "And it had the image of a dog tag on the top of it."That shirt sold well, but Boulos' first viral hit came by targeting a larger group: graduating high-school seniors. Boulos replaced some letters in "Seniors" with the numbers"15" (Sen1o5)and then came up with shirt after shirt based on that theme. One was a little rhyme: "Mighty and mean. So fresh and so clean. We are the class of 2015." It seems silly, but it sold, and sold, and sold. Boulos, who is 20 years old, was making more money than his parents.While Williams and Boulos are emblematic of a type certain type of Teespring seller, there are other sellers, like Kim Springer, who have built more of a brand identity on the platform. Springer has earned over $250,000 creating shirts that cater to the modern Christian woman, a demographic she believes has been underserved by the fashion community. Her first hit came with a shirt that read, "This girl still dates her husband."It's one thing to make a couple of popular shirts. It's quite another thing to sustain growth once people begin to crowd into a market that has virtually no barriers to entry. Williams credits his continued success to a disciplined system and his constant quest to automate parts of that systemthings he picked up while working for a time as a foreign exchange trader.He points to a shirt he sold that reads, "Florida girl in a California world.""I made $40,000 out of them," he says. "Do you realize there were 2,500 combinations of that shirt' I did about 500 before the bottom dropped out of it."This type of output requires efficiency, but Williams realizes that even if he continues to improve his system, he has to have an exit strategy.The Wild West days are overTeespring itself is changing. It is no longer an unknown platform, a Wild West with limitless opportunity for the right entrepreneur.More than 4 million people in the US bought things from Teespring last year. Ithas expanded beyond T-shirts into other kinds of clothes. In late September, Teespring began an expansion into Europe, and in August it struck a licensing deal with the NFL.The NFL deal is particularly significant. In the early days of Teespring, clever sellers flooded the platform with bootleg sports gear, much of which slipped through the cracks. Teespring itself made money on this. But instead of trying to sue Teespring out of existence, the NFL has decided to partner with it.Itrecognizes the company as a force.Amazon seems to have recognized this as well, and it has launched its own Teespring competitor: Merch by Amazon. "Merch by Amazon is an on-demand service that enables you to create, sell, and promote your custom branded T-shirts with no out-of-pocket costs to you,"the website reads.There is a race for this market, and Teespring is trying to stay ahead of it by diversifying beyond T-shirts, or similar items. The grand vision is to have Teespringwhich one can imagine eventually being called simply "Spring" become the place you go to when you want anything made on demand. You send in the design, itmakes it.Today it is T-shirts, but tomorrow, who knows. That is what investors like Andreessen Horowitz ($20 million) and Khosla Ventures ($35 million) are betting on, it seems.Teespring employees and investors have stars in their eyes, but what does Teespring's growth mean for early topsellers like Williams'One truth is that the easy money is getting less easy.Boulos had one of his designs ripped off by a copycat, who changed it just enough to not get it taken down. This shirt outsold Boulos' original design. And there are only so many easy subcommunities you can bombard with Facebook advertising."I've made a lot of money from Teespring, but I can't keep this up forever," Williams says. He plans to leave Teespring in 2016. "In the meantime, I'm taking those assets I have built, such as thousands of designs, and customer lists, and am turning them into a semi-passive income stream," he says.He is stepping away from the game.And when I asked Williams what his favorite T-shirt design was, he responded:"Hey, I'm a salesman. I don't care about the design as long as they sell."SEE ALSO:How one woman earned over $250,000 working from home after quitting her 9-5Join the conversation about this storyNOW WATCH: Animated map shows how railroads spread across America Click here to read full news..