1) When a user signs up for an account on the 2nd site, will an account also be synced/created on the Main website?

2) How many sites can be added for synchronization?


What about listings synchronization? Is that something you can think of adding?

1) There is no main site, all sites in chain are equal. If user register on 1 site, these data are propagated to other sites. Same for profile update or removal.
2) This is not limited
3) No, plugin is meant if you have car, real estate and dating classieds sites; or alternatively New york, Washington and Miami classifieds, then you want share user data across these sites, but aharing listing does not make sense.
As Importer plugin could be used instead.