If you are buying a used Mac there are many things you need to check out before you complete the purchase. You can run hardware diagnostics and check out battery and hard drive health. You should carefully inspect the screen, keyboard and other parts. It is also very important that you check for a firmware password and see if the Mac is connected to iCloud. The best place to buy is from Apple's refurbished store, but you can also find used Macs at some well-known resellers as well as eBay and CraigsList. Be very careful to avoid scams and protect yourself. Once you get your new old Mac, be sure to wipe it and start fresh.
Fabulous video full of lots of great suggestions. You're terrific Gary. I'd buy a used Mac from YOU sight unseen ;-)
One thing that I think should be checked too is whether the Mac you are looking at will handle the latest version of the Mac OSX. There are now models that are perfectly good that won't handle Mojave. I'm in that situation.
I buy mine used, usually from Apple.com