To be consistent with #114, which is the PR that removed the `preStart` methods, I have renamed the inner class to `InitializedSellerExchange`.

The `context` of the outer class is being used in various placed in the inner class. This is the result of using an inner class to achieve immutability instead of using `var`s to avoid the inner class, as we discussed through Skype. If we want to change how this is done, I think this should be in a new PR that applies to all actos, to avoid inconsistencies.