There are settings in member "groups" that make it look like we can enable users within that group to post their own files to sell. Apart from the permissions in the group, I can't find any information on this. Is this still a thing, or did it go away with the marketplace? If it is still a thing, how does it work/where can I find more information on it?

Yes, Downloads still allows users to sell files. You would need to have Commerce in order to take advantage of it. You can see the remaining configurations for it in Downloads settings and then you will also need to ensure that you have Payment Methods setup in Commerce.

@Jim M Thank you for your response. We do have commerce, since we sell a few things of our own. I have all the settings the way they should be, but I am still confused how/where people are able to upload files to sell. I don't see anywhere on the user end of things to do so. I must be missing something.

If you haven't already, I would advise starting with our guide on Downloads as this will walk you through how to configure the Downloads application, etc...: https://invisioncommunity.com/4guides/downloads/basics_447/introduction-to-downloads-r213/

Please let us know if you have any questions.