Buying second hand mac

Take a look at where your intended used Mac lands in terms of macOS support in the How to get old versions of macOS article. Apple has a three-year sliding window of support covering the current operating system that is in the Mac App Store, and the two previous upgrades. That means that Mojave (10.14.6) will drop off of support this fall when macOS next is released.

Microsoft follows Apple's product support window and in the Fall, they will require macOS Catalina (10.15.7) or later for installation of supported 64-bit Office products.

Things that should be done by the present Owner Before you take Ownership

What to do before you sell, give away, or trade in your Mac