Just where does the responsibility start and end?

This is addressed to not only the collectors out there, but also to the dealers who are also part of this site family. I need to solicit some opinions here. This is the situation; I sold a Franklin Half, 1963 NGC MS-64 to an individual through Ebay. Now he claims that he has not received the item and has opened a complaint on Ebay. I have provided him with a copy of the Post Office receipt showing the posting to him. Obviously he has not provided any proof that he has not received it. The auction listing states clearly that refunds are not offered. Because of the new Ebay regulations, it is not allowed to state in the auction listing that I am not responsible for the US Post Office delivery, and that optional insurance is offered, which is what I use to add, along with numerous other dealers. Okay, here is where you?all come in. What is my responsibility here? Am I to be held liable for the failure of the PO.? And if I have to issue a refund anyway, does this open the door for a flood of scams, ?I bought this coin and never received it?, which are impossible to refute? The buyer is claiming ?item not as described? which indicates fraud in my listing. But at the same time, he is emailing me with the claim, ?I didn?t receive it?. Am I to be forced to send all the items I sell as registered mail, with the additional cost. And do I assume this cost, or pass it along to my customers? What are the ethics involved here? Where does the responsibility of the seller end and that of the buyer begin? I would appreciate opinions from both sides of the deal. By the way, I sold the coin for less that $20.00. Money is not the problem, ethics and being taken advantage of, is. dean