Do not buy that Mac until its owner properly prepares it for sale. Refer to What to do before you sell or give away your Mac - Apple Support.
Assuming that Mac's hardware works properly, preparing it for sale is a very simple task. If the Mac's hardware is not working properly, you might as well buy a new Mac for what it would cost to fix it.
Buying a used Mac is a task best left to very experienced Mac owners. If you want to save a few dollars, buy one from the Apple Refurbished and Clearance page.