Spring (formerly Teespring) is an online marketplace where you can sell clothing items through print on demand method, which involves uploading your design, customizing your design on various clothing products, and putting on sell. I do not know anything about the popularity of Spring, however, back in the days when it used to be Teespring, it used to be a highly profitable marketplace. Since Spring is a marketplace and fulfilment company combined into one, you can try the platform.