I understand the seller's frustration - their means of supporting themselves has been taken away, but I'm not sure I buy the argument that we need the intermediary just to "add to the economy". To me it just seems like an unsustainable business model that was destined to die once global communication became much easier, not just because of the various treaties and agreements of postal services.
The moment that chinese sellers could get a site available to a global audience, the need for an intermediary vanished; before you essentially paid for the connection; an extra $18 on a remote controlled toy boat was paying for access to the seller's inventory, supplied by his Chinese connections.
Now, there's no need for that connection or that warehouse - the visibility for any foreign seller is much greater, even to the point that it probably would still be valuable even without the USPS trade deal.
It's unfortunate, but this just seems like a model that was destined to die.