Go see the actual unit, touch it, play with it, take your time.
Test all the ports, test all the keyboard keys, check wifi, check bluetooth, check charging, check the screen if any dead pixels, check battery capacity.

Very important check if there is a firmware lock enabled. Lastly, ask the owner to remove it from their AppleID (Find my Mac) before walk off with your new purchase.

Google any steps that you are unsure of.