Im going to step out on a limb and be honest here.
You don't need to apologize to this community if you accidentally sold something that you have rectified internally to the satisfaction of your customer. Internal policies of retailers and businesses are internal for a reason and you don't need to share those reasons with us as a whole. You have identified the problem and solved it, and you are implementing a policy that you will no longer sell non labeled accessories. Isolated incidents involving one or two customers do not require for you to say your sorry to people whom have never patron-ed you, and from a business standpoint it is bad business to do so.
(and sometimes there is such as thing as being TOO honest for your own good in business)