Hello Robert,
Interstate mail fraud is still a federal offense whether you are selling, buying, or trading.
You may want to collect all of your correspondence and contact info and offer your trading partner an opportunity to fulfill the trade before forwarding the info onto the USPS Fraud Unit.
It may be just a simple misunderstanding or an unforseen life-changing event that took priority to swapping a few strikes.
Best wishes for a amicable resolution,
Gene
dal166322