I sent 6 emails to him with no response.
That was the wrong action.
As you have learned.
Your only communication in a Dispute should be through the Dispute.
If I would have paid for him to send a coin back I know he would have sent back his low value coin.
And even if you have a No Returns policy you are entitled to demand the return of the item.
If he's gonna cheat, make him work at it.
You can have a No Returns policy, but you cannot have a No Refunds policy.
Also returning a different item allows you to appeal to eBay and -- possibly more important -- to start a Mail Fraud claim.