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Comment by Sambdala | Hacker News Reader

For most purposes, the risk of a double spend is negligible, and it's perfectly safe to accept an unconfirmed transaction.

However, there are still those purposes for which that's not a good idea...

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With no confirmations a double spend is easy (would require a custom Bitcoin client designed for the purpose). Is it likely? No. So, if I ran a coffee shop, I'd hand over the goods without confirmation. If I were selling a house or a car, I'd wait for a whole lot of confirmations. Digital goods probably don't need to wait, either, particularly for returning customers.

So, yeah, I agree...I just want to be clear about why the risk is low: Because most people aren't going to cheat you out of a cup of coffee.

To elaborate, a merchant can easily monitor (or pay someone to monitor) transactions being sent to miners in real time. The probability of a successful double spend decreases drastically as the time since the first spend increases. If someone hasn't tried to double spend in ten seconds or so, they'll likely fail if they try.

This is simply false. Many mining pools allow you to push to transaction to them. If your first spend has a 0.0001btc fee and the second has a 0.001btc fee, they will choose the 0.0099btc fee.

You cannot monitor transactions being sent to miners if they aren't propagated through the normal bitcoin network.

Along with that, there have been cases where mining pools have doublespent unconfirmed transactions and stolen lots of bitcoins. GHash stole 3000btc from betcoin https://bitcointalk.org/index.php?topic=327767.0

For in-person, retail purchases, double spend is a big issue since waiting around for 10-20 minutes while the transaction is confirmed is unworkable.

Smegger, you are hellbanned.

How would it be negligible? Seem like it can easily be done.

It's pretty easy to steal and use a credit card, doesn't mean a lot of customers do it. For low-value transactions, it's not worth spending much time on. You save that for low-volume high-value transactions.

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