Oh, and never buy a giant
Hard to beat their awesome warranty though
I have to agree with all of the above. give it a once over and if it looks like it has had a hard life then I'm sure you will pick it up pretty quickly first up.
Hold on rotating parts like wheels, cranks, fork, frame suspension, linkages, headset etc and check for any play of wear. Check that wheels and suspension move without resistance. Look for worn drivetrain but this has to be expected for a used bike.
You can always walk away - another one always pops up.