Do I need to repost my items in separate communities?
No! Items you post are automatically posted to all communities that are checked off in the Communities section when you post. Your posts may also be visible in some additional nearby communities, if you are in an area with multiple VarageSale communities. Here's a guide on how to post items, and here's a guide on how to choose which communities they'll be posted in.
Note: If you join a new community, your items will not be posted in the new community. You will need to edit each item to select the new community if you want to cross post there, since only you know which currently posted items you want posted there. :)