Quote from: cypherblock on June 13, 2014, 02:41:23 AM

Why is it not financially sane to do a double spend attack? If merchant is taking 0 confirmations, then there's probably like a 50% chance that my double spend will work. At checkout my evil wallet software simultaneously sends the bitcoins to the merchant address and does a separate transaction sending them to me. These are broadcast to the bitcoin network at the same time. One of these transactions will get added inside the block chain.

That type of doublespend is trivial to detect. Merchant waits 10 seconds and merchant (or payment processor) has a dozen listnening nodes geolocated around the world. Within the 10 seconds either merchants tx propogates the entire network or your double spend hits one of the listening nodes. If double spend is detected POS terminal reports "payment declined" and cashier asks you for payment in another method.

This isn't to say 0-confirm is safe but it will require a dishonest miner. Either one you pay for that service or mining power you control directly. This way the double spend is never propogated on the bitcoin network and there is nothing to detect. Of course if you are hiring 5% of the hashing power there is a 95% chance the real tx will go through. Of course a smart merchant before letting you pay by Bitcoins the first time will ask for a CC or check and then charge you the cost of the purchase plus $50 failed payment fee when your double spend works.