How to share posts (and plugins) between existing site and new, separate dev/test installation?

There are tools available for simplifying the synchronization of databases between multiple WP installations, such as WP Sync DB.

As @jdm2112 mentioned, using a copy of the database—versus attempting to use the same database—reduces the risk of development work interfering with the production environment. However, simply making a copy of one site’s WP database is generally insufficient for using it on another site. You can read more about moving WordPress here.