Factory Reset without password to Migration Assistant
I just bought a used MacBook and the migration assistant is still logged into the old user. How do I factory reset? So that way the migration assistant is no longer in their account
This is why buying a used computer can be a dicey proposition. Not only are you maybe buying someone's unsolved problems, but the laptop may not be usable unless this was done by the previous owner before they sold it to you:
What to do before you sell, give away, trade in, or recycle your Mac - Apple Support
The fact that there is an existing account already on there means the computer was not prepared properly. You should return it for a refund. If you want to try to make it work, you can follow the above link yourself, but if the laptop is connected to someone else's Apple ID, you won't be able to use it going forward without that previous owner helping you go through all the steps in the above link.