Yes, it is an option for users to upload their own items & services for sale - you can allow it on the listings app and the classifieds app (as well as the other built-in social networking apps in UltimateWB Full). Just the e-commerce app (which you get to from Configure Site > E-commerce) is for admin posting only. You can configure allowing users to post their items/services for sale for the Listings app, for example, at Configure Site > Social Networking > Listings , and for the Classifieds app at Configure Site > Social Networking > Classifieds .
If you do a CTRL-f search for "buy" on these admin configuration pages, you can jump to this section; here is the copy/paste from the Listings app config page (similar for the Classifieds / other UltimateWB Full social networking built-in apps):
You can read more about how to create Listing category types here: https://www.ultimatewb.com/listings-directory-app/create-customize-integrate-listings-directory , and for Classifieds here: https://www.ultimatewb.com/classifieds-app/create-customize-integrate-classifieds-section (similar process).
Online User Manual: https://www.ultimatewb.com/user-manual-how-to-guide