Easy solution: Require an in-person visit (as someone already offered) and escrow the *DEPOSIT* of 10 BTC, that way if they don't buy, you get 5BTC or 10 BTC or whatever the terms are. Nothing to lose, easy money.
so I get 10 btc put to escrow, I hold the rig and deny real life buyers to buy it from me, then they guy never comes through for whatever reason and I go back to my real life buyer and try to sell it to him but...opus.. he already bought one and has no money to buy from me... so I would lose the buyer and make 10 BTC.. I think not.