How do I set up simple user interfaces?

Hello,

I have a WP multi site and I am trying to create a website where users can buy items and also list their own items. I have Marketpress set up. I am now looking to create simple user backends, where they can post an item to the marketplace easily. Do you have any suggestions on which plugins to use to accomplish this? I have easy blogging plugin installed and it seems like the way to go. However I don't get add new posts(items) on the sidebar.

Please let me know if you have any ideas on how to make an easy to user seller/buyer backend/interface.

Thanks,
Brett