As long as you did not misrepresent item; its Caveat Emptor-Let the Buyer Beware.
He can try and sue you in his state, but it may or may not work- Depends on whether they view you as a business and doing business in his state. Some do some dont.
If he sues you in your state show up and state your case. If you are being truthful as to what occured and what you posted and printed in ad, then youd be fine.
If he can actually sue you in his state, and then you dont defend, then he could get a default judgement, but its unlikely that he'd be able to transfer in your state- He might try but its iffy.
So I wouldnt do a thing except to tell him what I told a clown who tried to pull a similar stunt with me. Tell him:
*The item is as represented.
*No refunds.
*Because the item is as presented its not mail fraud.
*A fraudelant claim suggesting mail fraud to coerce a seller into returning funds could be in itself a type of extortion and fraud- You will notify *Federal Authorities of possible extortion and Fraud.
*You will travel to his state to defend and will countersue.
*You will sue him for expenses, legal fees and punitive damages
*You will report his wife to the local state Bar as being an accessory to an Internet Extortion scheme.
*You will notify his local authorities of his harassment and extortion scheme.
I say Game On and crush him-
Since hes a Marine he should understand "You kill one of ours, we'll take down two of yours" Hit him with everything you have and promise me one thing; When he begs off, make him say "Mercy"
Scorp