hi McKay.
After you initiated tradeoffer, there also need second confirmation through Steam mobile application.
I'm building a user-friendly system that does it for the user (the user just clicks the "Submit Trade" button, and behind the scenes, initialization and confirmation occurs). I'm partially certain that a mafile is needed. But my users log in through Steam and that's it; nothing else is required of them. Some Chinese have already implemented this. I can't find any information about how this is done without the mafile secrets.
I'm asking here because you've worked with Steam much more than I have. Am I missing something?