Ummm,

These seem like questions you should research yourself these questions seem to narrow for the average game dev to know.

I know nothing about this platform, but I'll use brute force logic to answer.

1. I'd suspect they use their login system, giving that much control to the dev to customize and run their own login system defeats the purpose of having a single sign on
2. I'd suspect yes, anywhere money exchanges hands, there are laws and rules to protect both parties. oversight over this is not only common sense good practice, but is the legal and responsible thing to do.
3. No idea how hard it is, it's like any other game market place I assume, having the system automated defeats the purpose of quality control on the platform.