If they didn't hear from you for five days, and sent you a message saying "never mind, just cancel the auction", maybe you should've done that. If you sent the coins late, and they want a full refund and to keep the coins, they can go pound sand. They were coins, not Domino's Pizza. If they messaged you to give them a full refund or they'll leave negative feedback, you're golden; let them do it, point out the message to eBay, and eBay will remove the feedback. Don't screw up shipping next time. But don't let a buyer walk over you. What they're demanding is ridiculous. If you have questions, you can reach eBay customer service by phone; it might be worth talking to them if things go south.