How much collateral?
Is there anything beyond just the single contract.. or does each transaction build proof of trust?
The collateral is up to both parties to decide, both define it and send the proposal to the other party.
Example:
Guy from New York wants a guitar from a guy in Nigeria who wants 200 USD by bank wire. Both do not know each other nor they trust each other.
Guy from New York wants Nigerian counterparty to put 400 USD in Bitshares as collateral and agrees to bankwire the 200 USD.
Guy from Nigeria wants 250 USD in Bitshares as collateral from the guy in New York.
Deadline for agreeing to the completed transaction is set to august 1st. If not both agree until this date that the transaction went ok, both will loose their collateral.
If they accept this smartcontract and fund it accordingly they both can trust that the other party does not want to loose his collateral, so now both want the transaction to go well.
Scenario 1, everything ok:
Nigerian guy sends the guitar and New Yorker guy sends the 200 USD bank wire. Upon receival both give their ok (private key signature) for the multisig smartcontract to return the collateral to each.
Scenario 2, bankwire not received in Nigeria:
Nigerian guy does not sign the return of collateral and New Yorker guy could loose 250 USD in collateral instead of 200 USD bank wire. If he does not sign also the Nigerian guy could loose his 400 USD collateral. So he would not do it if he had received the bank wire in reality.
Scenario 3, guitar not received in New York:
Same thing, this time the New Yorker would not sign the return of the collaterals. The New Yorker might loose 250 USD in collateral plus 200 USD in bank wire, and the Nigerian guy would loose his 400 USD collateral.
With this system both want the transaction to go alright because they would loose more if they didn't agree. They can adjust the collateral as they want and how much their trust level to the other party is. The smart contract can also define payment in Bitshares or BitUSD upon agreeing on completion and returning of the collateral.
This is how it is done in BlackHalo. And works beautifully...