I've actually already created a role-based "Customer products" module that allows users to basically hook into the Ubercart system. Unfortunately right now it's pretty custom, but the overall idea was pretty straightforward. The only downside is then you have to figure out which parts of Ubercart you are allowing the users access to, such as File Downloads, shippable items, etc.

But basically what we're allowing on our site (in the near future) is a user signs up for a program, kind of like a reseller but selling their own products... they sign up, at which point a Role is assigned. The Role gives them access to a "Create Customer Product" screen (just an "add product node" form) and they can use the Drupal system in that way, to create an actual Product node type. This way it utilizes all of the same Ubercart features such as the Shipping modules, and basically IS a product being sold by the UC system.

I have some work to do with regard to what the users are allowed to see, and also assigning them a Taxonomy term from the start (we don't want the Customer products showing up in the Official Catalogue, for instance). But if you are interested in getting started on your own I can describe my process, and / or post some code.

Once I have time I'll probably abstract my code to something much more open, something like uc_customer_product or something, but right now not all of the pieces are in place with Ubercart core, as far as I'm concerned. I'd love to discuss this more if the Uberdudes are interested in seeing what I've done.

(It uses some of the same ways you import custom product nodes from another system like e-Commerce..)