On Thu, Jan 30, 2014 at 12:15 PM, Chuck <chuck+bitcoindev@borboggle.com> wrote:

In arbitration the merchant could argue the transactions seen on the network were insufficient.

The arbitrator would presumably have some rules about what is or isn't an acceptable form of payment.

HTTP has response codes for submission of the Payment message. We could add signing to PaymentACK and other things in future, if that turns out to be insufficient in practice.