Oh wow I feel for you! But here's a lesson. Always, always open box and inspect product carefully before refunding. What your buyer did is called "bait & switch". Another variation is to return the item but damaged. This happened to me. I returned the item to the buyer and told them I could not accept a return as product was not in "same condition as sent." I also told them I would pass the matter to my lawyer if I heard anything more of it. I didnt.
Your best route seems the legal one.
Here in UK we can frighten buyers by issuing an online sourt summons that is quite cheap to do. Can you do that in USA? Or get your lawyer to write him.
What about the USPS fraud/theft investigations team? USPS take mail fraud very seriously.
In my experience Americans tend to shy away from any kind of official "investigation" more so that we Brits.
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