Buying a Used Macbook, iCloud
Mac do not have hardware activation lock. Read the last paragraph of the "FAQ" for confirmation.


https://support.apple.com/en-us/HT201365


You would want to check for a Firmware Password which might need another question if you don't know what that is, but iCloud cannot prevent you from reinstalling an OS or activating any Mac that has shipped thus far.


https://support.apple.com/en-us/HT204455


Since a bad actor could use the lock function to remotely set a Firmware Password, your step of logging in and signing into your iCloud and successfully activating Find My Mac and locking it would ensure you have exclusive control of that device going forward.