This is an interesting idea! Some thoughts:

Doesn't acausal trade (like all trade) depend on enforcement mechanisms? I can see how two AI's might engage in counterfactual trade, since they can simulate each other and see that they self-modify to uphold the agreement, but I don't think a human would be able to do it.

Also, I'd like to hear more about motivations for engaging in counterfactual trade. I get the multiverse one, though I think that's a straightforward case of acausal trade rather than a case of counterfactual trade, since you would be trading with a really existing entity in another universe. But can you explain the second motivation more?