Have them send in a support ticket. Sometimes they might have obtained a stolen item some how and it might have been done in a recovery. However, in recoveries, if they paid something, the trade had been reimbursed and they got exactly what they paid or traded for the item.
Either way, it might be good for you to have them send in a ticket just to make sure no one's taking anything from them. They should also change their passwords for good measure and make sure their email address is still the correct one. ^^