I bought a PS3 from Cowboom about a year ago. There's a Cowboom thread here on CAG that you should check out, but to summarize it really quick, you're usually safe with grade 6-7 consoles. Just read the descriptions carefully. Scratches with grade 6-7 items I've read are typically non-existent or unnoticeable unless you're really trying to look for it. My PS3 was a grade 6 and came flawless. The box was a bit beat up, but I couldn't care less about that.
As for the warranty, up to you and your own peace of mind. I think there's a 30 day return policy, though I could be mistaken on that.
