I've had a similar situation where I've sold something, and upon arrival it wasn't working according to the buyer. It was working when I sent it, but postage handling issues occur quite frequently. I did end up refunding the whole $50 since broken during mail = sellers fault, but whatever I moved on quite fast lol.
Did you sell it via PayPal? If you sold via PayPal, usually their policies place responsibility on the seller to resolve the issue i.e. refund or ask the buyer to send the item back (you cover the postage fees however). That is, if they provide proof of it not working, if they don't then you should dispute it with PayPal and they'll take your side most likely if the buyer is uncooperative.
If it was via bank account transfer or whatever, it's between you two to figure it out. Keep in mind that he does possess your full name/address, and if the item really was broken, then yes he could probably call the police/take you to court.