looking at buy MacBook that needs OS installed I'm looking at getting a MacBook, and the seller did a Factory Reset and said it just needs the OS installed. Is there anything I should look out for?
Really, DON'T.

The Mac O/S is licensed to be used with a big dongle that prevents it being installed elsewhere. That dongle is called "A Macintosh."

Yeah, wipe the drive before selling. that is what they call "A Good Idea."

Just remember that a Mac with no O/S on it is a Mac that you can't really run basic diagnostics on:

Does it have the hard disk and RAM capacities as indicated?
Does everything work? (EG all the ports, WiFi, monitor, etc.)

So why didn't they at least reinstall from the media that came with the Mac. or from newer ones, install from the COMMAND-R, internet restore? It doesn't take long. And failing to do that makes me think (correct or not) that they may be trying to hid a problem.

Yeah you can bring an O/S install DVD with you and do some basic checking. or even TechTool Pro and sit there for an hour while it does a comprehensive check on all the hardware.

But really, when someone is selling you a Mac with no O/S installed, just say no.