How is copyright protected for resale files?

Question

Copyright

Greetings, comrades.
A site is being created where users can buy or sell their files, i.e. made by them personally. A situation may arise, for example, when another user downloaded a file, modified it a little, and began to sell it as his own.
Or downloaded somewhere, began to sell, and a real author appeared.
Due to circumstances, digital signatures, as well as various kinds of watermarks, are not suitable for files, in any case, the file can be modified, made in another program, etc., etc. Moreover, the file is, roughly speaking, the source. That is, a real author can do it in days because of his experience, and a deceiver can do it in a couple of hours because of his experience.
How to determine who is the real author?
Maybe someone will suggest a way how to solve this controversial situation or to protect the authors as much as possible?
That is, with minimal nerves for users.
The price of the files is not high enough to be patented or anything like that.
What can be prescribed for such cases in the user agreement?
What actions can the site administration take if two users have almost identical files, and one filed a complaint against the other?
Also, if the case goes to court, who should sue whom? For example, the author with the site administration, and then the administration with a deceiver, or the author with a deceiver right away?
By the way, can you sue the site? Who is the accused in this case?
Site owner?
By the way, how is the owner of the site calculated in the event of litigation?
The one to whom the domain is registered?
And if the domain is a redirect to another domain, to Vasya Pupkin, who then?
Thanks in advance.
PS Open source rules :-)