Here is a Marketpress plugin that iMath coded up !
We call it Tradr Products.
It is a front end form that allows logged in users to add a product from the front end of your blog.
Please download, leave a review and comment on the discussion forums at the Tradr Products group.
Note: Until the new marketpress is released you have to enter your shipping details.
Please donate some points if you think this is cool. Cheers