# To backup downloaded repositories cache, open a konsole/terminal, and run (as root)
tar zcvf apt.tgz /etc/apt/ /var/lib/apt/ /var/cache/apt/
#This will create a file apt.tgz that contains all downloaded files and repo lists. Make sure Synaptic is set to keep downloads, and not delete after using, and this will bundle them all up.
# To restore downloaded repositories cache, copy the apt.tgz to user's /home folder, and run (as root)
tar zxvf apt.tgz -C /