OP:
Why don't you tell us what your budget is?
Something to check when buying a used MacBook:
Ask the seller to power it down, all the way off.
Then, press the power on button and observe what happens.
Does it boot to an EFI password? YOU DO NOT WANT TO SEE THIS.
It should boot either to the finder (no password) or to a user login page.
And then the seller should be able to log in.
Also, be sure that the seller has "signed out of" iCloud, iTunes, his Apple account, etc.
I'd recommend that you consider buying an Apple-refurbished laptop.
A little more money, yes.
But you can BE SURE OF what you're getting.
And you get a 1-year Apple warranty and it's eligible for AppleCare, as well.