@nibor209 is absolutely correct. If the previous owner is unable or unwilling to properly prepare the Mac for you, then you should return it for a full refund.
Even if the previous owner does perform the steps in that Apple article, I highly recommend you perform your own clean install of macOS as well. This will make sure there are no other nasty hidden surprises awaiting you....or at least minimize the likelihood of them. A clean install is when you erase the device followed by reinstalling the OS. How you do this depends on the exact model of the Mac....for most Macs it would be advisable to first create a bootable macOS USB installer while you can still boot the computer. With 2018-2020 Intel Macs, there are other things necessary for the prep work in order to even use a bootable USB installer.