How to allow users to upload and sell their products on my site while i get a cut ???? NEED HELP

CAN SOMEONE HELP ME ????? i just bought this Marketpress pluggin and im trying to open up a ecommerce business that will alway users to sign up and post their items to sale while i get a small cut. But im having a problem with the site, i cant figure out how to let users sign up, login, or post products to my site to sell. Im not trying to use Multisite i want all products on my site while i get a cut. Similiar to ebay. Is there anything you can do ??? Can you help me find a solution ???? Walk me through it ? Or send me to the right person to help me