My understanding of the SCP is that when a trade happens it must hold the same value across the network. If a transaction happens that is not a 1 to 1 exchange and is validated by a node it is technically a befowled node. Would there be a way other then using an authorization revokable coin to make this physical transaction work?
Stellar transactions are non-reversible. A solution to prevent fraud when buying a physical product is to use multi-signature and have both the buyer and the seller agree on an impartial mediator. The principle is the same as Bitcoin 2-of-3 multisig transactions.
The developer documentation has an example on how to set up a multisig accounts on the Stellar network