Buying used Macbook Pro, checking if it's linked to previous owner
There is no link you can’t undo or clear to an old account other than Activation Lock and a firmware password. 


https://support.apple.com/en-us/HT208987
https://support.apple.com/en-us/HT204455


You need the owner to give you a bill of sale and cooperate with both the above procedures. Get their support in writing or escrow your funds till you can validate things. 

All the other steps for preparing a machine for sale protect the seller and don’t inhibit your full use and control of the Mac when you take possession of it. You will want to erase the Mac and be sure you never lose your firmware password since you can’t bring the original proof of purchase and prove the Mac is yours to have Apple unlock it. 


https://support.apple.com/en-us/HT201065