Script that lets users sell their own assets like the envato marketplace, graphicriver.net .

Just want to focus on users selling designing template part which includes:

Business Cards,Stationery,Flyers,Brochures,Newsletters,and Resumes

Selection made by buyer has option to modify via site(using flash base realtime)or pay download using their own software. Once selection, payment and download has been completed buyer can make additional product purchase for printing(ex;the printing for Business Cards,Stationery,Flyers,Brochures,Newsletters,and Resumes. Graphic River allows buyers to use uprinting for printing need, but I would like to keep it inside of site as an additionall function.