You should check the following things when buying a used Apple Macbook laptop.
(i) Proper Warranty. You can check warranty status through apple website.
(ii) User level scratch. How much scratchy it is. I don’t expect flawless but scratch level should be minimum. Specially check the LCD screen very well. Any scratch on the LCD will make it ugly.
(iii) All the hardware are correctly working or not. Specially sound, optical drive, trackpad,keyboard etc..Since it is a second hand macbook, so there is ample chance of having hardware failure.
(iv) Check out whether all the accessories are genuine or not. If there is not genuine accessories inside the package then don’t go for it. Check for all the original papers along with the original bill.
(v) Check out the battery life. Check whether battery is correctly charging or not. Check whether there is a significant drop of battery level or not.
(vi) Restart your macbook twice. If it is opening and closing perfectly then there is no risk of buying it.