One of the great selling points for Macs has been a combination of their longevity and resale value. I know plenty of people with decade-old Macs. In the last two decades, I’ve easily gotten seven or eight years out of some Macs I’ve owned, and then sold them to folks who kept them in service.
There’s a lot to consider when buying a used Mac to make sure that it will keep working. But something that you might overlook is that security decisions made by the previous owner could conspire to lock you out in certain circumstances. This could happen on a restart, when trying to erase and reinstall macOS, or even on logging in, depending on what state the Mac was left in when you purchased it.
