I am future T-Shirt seller. I don't want to have problems with copyright for my designs. So I have made some research and learned that parody is fair use.
Can I sell below parody T-Shirts? (Parody of Superman, and Parody of GodFather).
Can I sell below parody T-Shirts?
Sure, you can sell those shirts. But there is no guarantee that your own determination and claim of Fair Use for a parody would prevail in court.
The determination that your use of those derivative designs - Fair Use as a Parody - is not yours to make; it is for a court to make, if and when the copyright holders of those two franchises choose to take you to court.
Your determination of Fair Use could only be valid in the US; there may be no Fair Use law in your country, and any relevant international laws and agreements will impact what the legal steps the copyright holders can take against you. Read Fair Use again for exceptions to the law and about international laws such as as Berne Convention and World Intellectual Property Organization.
Warner Brothers (Superman) and Paramount (Godfather) could decide at any time to confront you with a cease and desist letter or a lawsuit to challenge your claim of Fair Use. They may win; you may win your defense of their lawsuit. That's a risk you will take.
The fact that other people currently produce shirts with similar designs doesn't mean anything. They may not have been confronted by the copyright holders; or the copyright holders may not care to confront small producers of possibly infringing designs and products.