I agree with the others that this is not the seller's fault, and that it was likely a mistake on your part to mail the item back prior to agreeing with the seller to the terms of a refund. Yes, it would have been better if the seller wasn't rude to you, but if a crime was committed, it was identity theft by the adolescent, not improper seller behavior. I think your anger at the seller is misdirected.
Sellers on Ebay are not required to accept refunds. Did the listing for this item say the seller accepts return? If not, you are out of luck as a buyer unless something is wrong about the item itself, like that the seller badly misrepresented the condition of the listed item. Ebay tends to side with buyers in these dispute cases, and many sellers have tales of being shafted by dishonest customers.
If a seller does accept returns, they still don't have to automatically accept returns in every case. If the item has shipped, the way things usually work is that you get refunded the item price only IF the seller accepts returns, you've submitted a return request through Ebay's system, and the request has been accepted by the seller, after which you mail the item back. You typically have to pay shipping, and sometimes a restocking fee as well, for returning an item. Similarly, if the item has not yet shipped, the the seller is not required to cancel the transaction.
From the perspective of Ebay, a transaction was duly and fully completed by the seller. Then entirely separately and outside of Ebay's system, you mailed the seller a present. Your perception of the seller's wealth has no bearing on them owing you anything.
In this situation I would be furious with myself for being lax about password security, and with the adolescent for stealing from me. I 1-up the suggestion about the adolescent working off their debt to you over a week of Sundays. Aside from the monetary element, that young person needs to learn for their own sake that what they did was wrong and that actions have consequences. Long term, I think you'd being doing them a favor by taking this identity theft seriously.