Multi-vendor marketplaces, like ThemeForest, can be very successful. Chose a niche and create a vendor website for it. Your marketplace could be anything, from a platform for local artists to sell their work on, to an online digital product store. Once set up, invite people in that industry to sell their products on your site. You take a percentage of their profits when items sell.
There are numerous printing companies that will print your designs onto not just T-shirts, but also hoodies, hats, posters, cushions, bags, and phone cases, to name a few items. These businesses also offer fulfillment services. This means that you promote the merchandise on your own site and once you sell a product the printing company will not only print your design but also send the item directly to the buyer. So all you have to do is concentrate on the marketing and selling of your products, and the fulfillment company will take care of the rest.
In 1994, Tobin launched a beta version of PC Flowers & Gifts on the Internet in cooperation with IBM, who owned half of Prodigy. By 1995 PC Flowers & Gifts had launched a commercial version of the website and had 2,600 affiliate marketing partners on the World Wide Web. Tobin applied for a patent on tracking and affiliate marketing on January 22, 1996, and was issued U.S. Patent number 6,141,666 on Oct 31, 2000. Tobin also received Japanese Patent number 4021941 on Oct 5, 2007, and U.S. Patent number 7,505,913 on Mar 17, 2009, for affiliate marketing and tracking. In July 1998 PC Flowers and Gifts merged with Fingerhut and Federated Department Stores.