OK, I think I've got it.  We want *both* a model *and* a module.  Here's how this is gonna work:  we're gonna have a Quartermaster model, and it knows things like how to ask whether a buyer can buy a thing, a seller can sell a thing, and handle the transaction itself.  Then we're gonna have a Transactionable model that implements the methods Quartermaster needs to use in order to handle its logic.  I'll write up an example of this later this evening.