I would have the seller boot it from being turned off. You want to hear the chime and watch it bootup. If there are any passwords, have the seller give them to you to enter yourself and then write them down. A MacBook can be locked with a firmware password that is not easily bypassed if it can be at all so if it is on there, make sure you get it so you can remove or change it later. Once booted, go to About this Mac and verify that OS version, year and model, and RAM are what was advertised. Then click on System Report, go to SATA and check the capacity and type of hard drive. Then go to Power and look for cycle count and battery condition. Low cycle count is good but battery condition must be normal or you will need to replace soon...