Charles is right. Under Ebay's rules, a seller guarantees delivery (not just "sent") no matter what level of postage is used. The lack of tracking goes against the seller, not the buyer.

Some of us sellers used to charge extra for insurance, but even without insurance Ebay told us we were responsible for the safe delivery of all purchases.

Federal law does not provide this protection - the terms & conditions of the seller's contract with Ebay covers it. In addition, you may have rights under the buyer's PayPal agreement or credit card rules for undelivered merchandise. In the old days, the law would have left you without a remedy but modern commercial rules have filled the gaps.

As for foreign transactions, I believe the Ebay rules in other countries also protect buyers the way they do in the U.S., but I have not actually read those terms.