In most cases, for low value transactions, zero confirmations are acceptable.
Not any more. Since the transaction malleability problem, the trend is against accepting transactions without confirmation.
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There are many circumstances where accepting a transaction without confirmations is acceptable. There are other situations where waiting for confirmations is not a burden. If your needs don't fall into either of these categories, then you need to look into third party services that will guarantee faster transactions.
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As long as you have a trust relationship with the payer, or a very high ability to repossess the product or service that was exchanged, there is little need to wait for confirmations.
Furthermore, there are many transactions where the profit margin is high enough and the fraud risk is low enough that 0 confirmation transactions can be accepted.