Mail to Russia can take 4-6 weeks or even longer and unless that is made clear to a buyer then often you will receive these sort of disputes.

If the item has not been sent with tracking then there is a very good chance you will lose the case unless the item happens to turn up in the meantime.

If the item turns up after the case is resolved against you then you will just have to hope that the buyer is honest and offers to repay any money that is refunded.

The only advice I can offer is that you communicate with the buyer explaining about long postal delays being normal for that part of the world and that it has probably been held up by customs and hopefully get them to be patient in the meantime by reassuring them that it should turn up eventually.

Also try to reassure them that although it was sent without tracking, the item was definitely sent. Any proof of postage details that you have such as a postal receipt showing the parcel sent to Russia at the time you said it was and/or a customs registration number from the green customs form may help allay their concerns.

Unfortunately the way the ebay & paypal dispute systems work if this person is deliberately trying to scam you then you have no hope of winning.

If they are simply genuinely concerned, however, then delaying them from escalating the dispute to a refund claim may give the item time to turn up and then they should simply close the case themselves.

Good luck.