You can find some great deals in used computers on eBay and Craigslist. For any used computer, the first priority should be safety. The vast majority of used computers are safe, but you don’t know how careful the previous owner was. Are there risky files or software on the computer?
The operating system needs to be young enough that it will continue to receive security updates. Windows XP and Vista no longer receive security updates. And Windows 7 support ends in Jan 2020. You could opt to pay extra for a Windows 10 update for any of those older operating systems. You might also opt to forgo Windows and switch to Linux which can be a good option if your programs have Linux versions.
Apple typically provides security updates for approximately the last three versions of macOS. If the computer isn’t too old, it may be upgradeable to a newer macOS.
If the used computer has Windows 10, the seller probably reset Windows to its factory condition. This is an easy way for the seller to clear out their personal files. This is often done from a separate “recovery partition.” It would be wise for you to reset it again.
But first, make sure this is a legal version of Windows. Choose Start, Settings, Update & Security. If you see the message “Windows is activated with a digital license,” this is should be okay. If there is a 25-digit product key number, write it down on paper. You’ll probably need it.
To reinstall Windows 10, choose Start, Settings, Update & Security, Recovery. Follow the instructions. Choose “Remove Everything” and then “Remove files and clean the drive.” This will remove software like Microsoft Office or Adobe Photoshop. That software will need to be reinstalled if you have the disks, media, and licenses. Without these, the software will be deleted.
This can take a couple hours. Then you’ll want to install security software and run Windows Update. If the reinstall added bloatware or other unwanted software, you can uninstall these from Start, Settings, Apps & Features.
These steps make a very safe computer. Some people go even further and discard the hard drive that came with the computer, then install an entirely new hard drive.