Bitcoin nodes will reject transactions that conflict with ones they have already seen. Once the transaction is broadcasted, the only way to replace it would be

Bitcoin nodes will reject transactions that conflict with ones they have already seen. Once the transaction is broadcasted, the only way to replace it would be to mine the next block and include it yourself. The only other feasible way to double spend a merchant would be to scour the topology and find a client that has low latency to the merchant you are trying to defraud, but much higher latency to any significant hashing power. Unless they have their node configured to accept incoming connections, which is not recommended for merchant nodes, the attack is very difficult to pull off and requires a significant element of luck to work. There are also companies like BitPay that will absorb the risk of a double spend in addition to handling your payment logistics, for a fee less than the typical credit card fee.