Is posting content on more than one site possible?

Just trying to get my head around how this might work.

1 - A user logins in via Facebook to one site on the network. Is it possible for them to add a new post to other sites on the network?

2 - Is it possible to grant them permission to post on another site on the network?

Similar situation using the Classifieds plugin.

3 - If we have this installed across the network and someone logs in on one site to post an advert, can they select to post their advert on more than one site?

4 - Or, if access granted they can post the same advert individually on another site without having to login to that site separately?

I hope that makes sense, if not let me know and will try to explain better.

I suppose essentially what we want to be able to offer as openly as possible is to allow any user to post on any of the sites across the network that we allow if it all possible?

If not, are there any ways to work around this or what other options are there to achieve something similar?