Yes, I agree with everybody else. Tell him to keep them, or send them back for a refund. And DO NOT refund him without getting your product back. He's just being prick, trying to get them for free. I doubt he'd even send them all back if it came down to it. Probably already installed one on his car.

And even if he sends them back, your not liable for any additional shipping. For the minuscule amount of rust, i'd tell him to go fuck himself. As for the feed back, screw it. It's only one negative comment - won't affect you that much. Just dispute it.

On the flip side, how many do you have left? I'm sure somebody (possibly myself) would buy the remainder.