Teespring is a marketplace of consumer designed products. Initially, it was started as a market for customer designed t-shirts. It offered customers a way of selling self-designed T-shirts for fundraisers. They provided a great DIY online design tool for customers to create professional looking T-shirts and other printed garments. Later this evolved into a marketplace for entrepreneurial designers to sell their custom designed T shirts to raise funds for themselves or charity of their choice.
A number of Teespring clone websites like Bonfire, Sublimator also offer slick online t-shirt designer and ability to print a limited number of custom T shrts. Recently all of them have added more and more custom print products like hats, other garments etc..
If you like to create a website smilar to Teespringa and clone their business model, you can do so using iScripts Printlogic. Check out a live demo to see how it looks from the user perspective and admin perspectives. Some customization might be required, but the majority of the functionality required is available off the shelf product.