I think the OP regrets his not listening to the advice he received in December. What he is doing here now is looking for advice on how to proceed, though that is not what he said in his opening post. He is likely more angry at himself than eBay.
@28wils
As for how to proceed, I would send a registered letter to the scammer and also an email. However you have to contact them. Let them know politely, and without vitriol, that you intend to bring the situation to the attention of the Postal Inspectors, the police in their and your jurisdictions, and that you will sue them as well. That is, unless they send you the ring or the payment.
Scammers hate being on notice with the law. This is less likely to work if you have already sent angry diatribes, but it is worth a shot. Be persuasive, don't harass, keep your cool. Let them know that the Postal Inspectors will be particularly interested in them during an investigation into the crime, as it is a federal offense to use the mail system to defraud. (The United States Postal Service is a federal agency. Because of that, the law categorizes mail fraud as a federal offense. The potential prison penalty for such a crime is very high. Each offense can result in a sentence of up to 20 years in federal prison.)
This is all assuming the scammer is domestic and not international. It may not be possible for you to actually sue the scammer in Small Claims court, but they won't know that for a fact even if they suspect you won't act. But the whole idea here is to convince the defrauder to return your ring or make payment because it will be too risky not to. Keep in mind, being that a month has gone by, they may no longer be at the address you sent it to, so the effort may be in vain. But it is better than doing nothing.
As for your blaming eBay, they cannot protect sellers from themselves, though it would be better if they made these scams more well known to new sellers. However, even then, eBay would still have to rely on members to read the User Agreement and policies. Those policies are eBay's way of helping to prevent fraud on their platform. There is just no way for eBay to save sellers from a lack of knowledge, or impulsive behavior, while using their service.
I wish you well. Good luck.
