It's obvious the answer is they accept 0-conf/zero-confirmation, but most likely they do/check these things (since there's few second delay on the video) before the system decide to verify the transaction :
1. Check fee in the transaction is sufficient to get confirmed
2. Check the bitcoin network if there's double-spend attempt
3. Broadcast the transaction to many other nodes to reduce double-spend attempt chance
But, there shouldn't be any problem with double-spend attempt if the "attack" transaction is broadcasted few seconds slower. More info https://bitcointalksearch.org/topic/m.3819
Topic: How do these Bitcoin transactions confirm so quickly? (Read 225 times)
This video was made in October 2017, and the guy was shopping in a Tokyo supermarket, and paying with Bitcoin.
https://www.youtube.com/watch?v=W4J_myvNYG8
He seems to have had his transactions confirmed almost immediately, and I don't understand how this happened. I can see it working with the lightening network, but I don't think that was available then. Is it just that the shop is prepared to take a risk, and as long as the transaction was sent, they don't bother to wait for a confirmation? That seems to be a bit risky, as if it was sent as a no/low fee transaction, there must be a risk that it is never confirmed.
https://www.youtube.com/watch?v=W4J_myvNYG8
He seems to have had his transactions confirmed almost immediately, and I don't understand how this happened. I can see it working with the lightening network, but I don't think that was available then. Is it just that the shop is prepared to take a risk, and as long as the transaction was sent, they don't bother to wait for a confirmation? That seems to be a bit risky, as if it was sent as a no/low fee transaction, there must be a risk that it is never confirmed.
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