Selling on old standard car stereo. Is code required for buyer?

Im looking to sell on my old standard RCD500 car stereo as it is no longer reading cds. I noticed when purchasing a replacement on ebay that a code is generally required with every genuine car stereo or at least with standard fits anyway. Can anybody provide me with a bit of information on where i might retrieve this code? I will be selling the stereo as broken, so more than likely somebody will buy it for parts, or at least have a go at repairing it. I dont want to stick it up online unless i'm providing all of the relevant information on the item and know little to nothing about this code and what it is. Any bit of information on this would be greatly appreciated. Thanks.