I don't do these sort of trades but if I did I'd try: Buyer sends 1/4 of the BTC Sellers sends 1st card Buyer sends 1/2 of the BTC Seller sends 2nd card Buyer sends 1/4 of BTC That way the risk is shared, and both parties stand to lose at most 1/4 of the value (if seller defects after receiving the first payment, or buyer defects and fails to make the last payment).
Noterized letter of intent works too for domestic sales. Not anonymous but keeps people honest, and gives you option of small claims court.
When I was 18 and needed to get a letter notarized, it cost me $35.