After a site with similar concept to teespring.com but for a different product market. Please view the site so that you understand how it works.
Create a Campaign:
User can choose from a range of products and create a design for it.
Buyer would have the option to purchase the product with the design or to edit the design themselves.
This is just a basic overview - please let me know if this is a project you can complete and we can discuss it in more detail