For example, if you had someone designate a long-term Bitcoin address, that would serve as their only destination point for funds, and you as a vendor would accept transferring their funds to that address, I believe you could begin to find the outline of a secure payment system.
If you are trying to find a way to let a buyer prove that payment was sent to the seller, then you might want to know about the payment protocol being built into the Bitcoin-Qt/bitcoind client.
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https://bitcoinfoundation.org/blog/?p=85