Usually they can just register it to themselves and it's unregistered from you, like when you sell a car. But yeah, just give them a call because AppleCare is fully transferable. So is hardware, if there's any warranty left on it and AC is not purchased. Software I wouldn't worry about. Generally it's not good to sell software, but if you delete it from your computer first (or if you sell it with the computer you are selling) it's ok.