I can't migrate from time machine - Migration Assistant asks for password to non-existant user

if and only If you are not 100% certain the Original Owner has NOT done the below steps, you have two options IMHO

1 - Return for Full Refund

2 - Perform these actions yourself

link below

What to do before you sell, give away, or trade in your Mac - Apple Support (CA)

The " Owner Account of this computer "  has that privilege to add, remove, upgrade or downgrade accounts. This would include adding more Admin Accounts and / or downgrading an ADDED Admin account 

Set up users, guests, and groups on Mac

Normally the " Owner Account  " is the very first Account used when the Computer was Very First Setup as New and out of the box.

Adding additional Admin Account can be done there after.

Below link all provide additional insights about the Difference between Admin Accounts and Owner Accounts  

https://eclecticlight.co/2021/07/18/last-week-on-my-mac-the-perils-of-m1-ownership/

You are welcome and thank you too

Do come back to the Apple Support Communities ( ASC )  in the future, if / or when additional questions may arise