How do I send and receive privatemsg?
I'm pretty sure that using Services Entity API  will provide what you are looking for.  An endpoint shows up for me after enabling the module.

This module provides support for all Entity API entity types to Services. All entity types get a standard resource, similar to what Services provides for code entity types such as nodes.
This module allows to get entities by properties or fields and also paging and related entities that are defined properly.