If a "MacBook", then I'm guessing used. Apple hasn't sold new MacBooks (at least with that name) since the 2010 model was discontinued in July of 2011.
Of course, what you check for depends a lot on which MacBook you are considering.
There's 7 to 10 different MacBook models, depending on how you count them. (and that does not include any MacBookAirs, or MacBook Pros)
If an older MacBook (2006 or 20007, etc), then the battery might be an issue. The hard drives were smaller capacity then, and the plastics on the case can have various (mostly appearance) issues.
As with ANY used laptop, not just Macs, you want to check the screen for condition, hinges for smooth operation, magsafe adapter cable for serviceability. And, of course, on a used Mac (ANY used Mac) you should get a disk that you can use to reinstall OS X if that becomes necessary (many would say that your first task, after purchasing a used Mac, is to erase the hard drive, and reinstall OS X.)
It would also be nice to get a used Mac that has the maximum RAM memory already installed.
So - which MacBook are you looking for?