I was wondering if I should start backing up the local db directory for my packages? My current method has just been to save a output file of ( pacman -Qs ) on a separate usb stick.
But recently I went ahead and ran ( tar -cjf /var/lib/pacman/local )
and put it on the same usb stick.
My question is should I backup the sync as well? What is it used for?
The ( archlinuxfr.db, community.db etc... )
And what would be the correct way to place my local directory back on the system if the time came? ( tar xjf / [or] tar xjf /var/lib/pacman ) what is the correct way?

pacman won't run without sync - it is the database for the repos.
don't know if there is one correct way, probably ways that work better than others - I'm unfamiliar with the tar methods you mention as have only used rsync to backup and restore system.