Hi,
Please visit our store www.xteas.com. The big picture is to have add Buy & Sell section.
SELL section will allow designers to design their t-shirts, by choosing the tee color, front or back, selecting font or images and placing them within print boundaries. If its a seller, then the design will go internally for approval before publishing on the storefront. If its the customer then he pays & checkouts.
In the BUY area, similar option to create you own t-shirt, or buy the one showcased (grouped by artist, style, color, etc).
Layout as of http://www.hoodieremix.com/#design
Store Format as of http://www.zazzle.com