I've seen so much grief here over used Macs said to be "ready for sale" that I cannot ever recommend buying used.
If you must do this, get a written guarantee stating you get all your money back if the computer is not properly prepared for sale and reuse. Even then, such docs are not very easy to enforce if the seller balks, and your resulting legal fees from attempting to get your money back could buy your a really expensive, top-of-line new Mac new.
Mac are in high demand worldwide and that fact has opened fertile ground for shady sellers ready to cheat over-eager buyers. Your safest action is to run away. 🏃🏼♂️