Reinstalling Android on FP3 in order to resell

Hi, I installed /e/ on my Fairphone a while back.
Now I want to resell it, and therefore reinstall Android.
As I have no experience with OSs I would like to know how I would go about it.

Greetings and thanks in advance, Carl

“This article will also help you to revert to Fairphone OS if you installed a different Operative System (OS).”

Thanks, but I have trouble unlocking the phone. The phone remains locked even when I activate “OEM unlocking” in the developer options.

This switch only allows the unlocking process.
You still have to do the actual unlocking, just continue following the instructions.