I think the best solution is to use uc_roles... user purchases "Product creator" role > checks out > On successful checkout, user can now create content to be purchased.

One question on the logistics of this, upon successful payment, should the node/product be available immediately to other customers?

Should the user who is able to create content need to purchase the role each time they want to submit a new product, or should they be allowed a certain number of nodes they can create? (This is how ec_paypernode works - which I'm looking at to see if there's any code I can recycle).

It wouldn't be too hard, I think, to give a certain number of allowances, and attach the role to the permission, and only allow users with that permission to create products. One thing it looks like ec_paypernode doesn't do is attach the user's created products with their transactions. (Not sure if that means their transaction which allowed them to create products, or the subsequent purchases from other users). Fortunately with Ubercart that would almost take no effort whatsoever.

Let me know your thoughts and I can perhaps start tinkering!