The same opensource app, downloadable on both stores but paid on playstore and free on fdroid. Is it legal and is it ethical? Why?
If you have the rights to publish the software, then you can do that. Why would it be illegal or not ethical? If you have the rights to, then you can even publish your software with different license on both “platforms”, if you want to.
Do you want publish your own software or do you think of taking an existing Open Source software and sell it on PlayStore? It would be good to know what your goal is. Or is this a question of curiosity?
Then you can do whatever you want with it. You can publish it under different licenses or charge money on one platform and not charge any money on another. Its all up to you and its legally right and ethical. Some people don’t like that you charge money for an Open Source program, but that is their decision. Its totally okay to charge money for something you created. Don’t let people dictate you how to publish and sell your own software. If someone does not like paying and supporting you, you can always point to Fdroid or the source code. Absolutely ethical.