"You hereby claim that you are either the original content creator or have written permission from the original content creator to sell this course. You agree that if you are judged by us to violate another creator's copyright, you will be liable for double any revenues generated by the sale of the course, payable directly to the copyright holder. You will also be permanently banned."
Or something to this effect. Make it part of the EULA that copyright violation won't be tolerated.
Better content moderation is one step.
Another? Add a part of the EULA that states:
"You hereby claim that you are either the original content creator or have written permission from the original content creator to sell this course. You agree that if you are judged by us to violate another creator's copyright, you will be liable for double any revenues generated by the sale of the course, payable directly to the copyright holder. You will also be permanently banned."
Or something to this effect. Make it part of the EULA that copyright violation won't be tolerated.