How can i sell files using ubercart
There is already module File downloads in the ubercart which allow user to sell file with the product.


Install the module File downloads from the module list.


Assigning directory to file donwload:


Go to Administration -&gt; Store -&gt; Configuration -&gt; Product 
In the File download settings section, set the path of the directory of the files and save the form.


Uploading files:
- Go to Administration -&gt; Store -&gt; Products -&gt; View file downloads
- Upload files into directory (which you have configured in above step) using upload file action.

Adding files to product:


Edit your product and you will get the feature link in top of page. click on this link.
In feature form, you can add file download feature to the product and it will redirect you to form to assign file to product.
Select file name in File download text field. This is autocomplete field, so you need to configure correct files and save form.


When the user purchase product and pay the amount, it will send a link to user in his mail to download the files.

Regarding your second question, there is module Ubercart Marketplace.


  Ubercart Marketplace adds multiple seller support to the Ubercart
  module. It includes a series of modules that limit the access to
  Ubercart functions for sellers. Additional functionality is added to
  support this from both a marketplace seller's point of view and the
  store administrator's view. Sellers (with the proper access
  permissions) gain the ability to view their products, track and
  fulfill their orders, and monitor their sales reports. Store
  administrators can view seller reports based on seller order statuses
  and seller sales. Support is included for Paypal Mass Payments, with
  which an administrator can automatically pay his/her sellers for
  custom periods.


Kindly see that Ubercart marketplace is still in development version for Drupal 7, so may be there could be some issue in this module.