You can actually use Ubuntu Software centre for taking Screenshots as here
Also you can see it in Synaptic Bottom left list selections.
cd /etc/apt/sources.list.d/ && cat *.list you will get list of all repositories in your system.
You can also you Y PPA manager to backup and update your repositories.
[Since this method involves installing all the packages , but if we think that newly Installed OS will already have those 90 % packages , so it will ignore the present ones and install the new manually added ones , which will be only your manually installed applications]
This method from here.
sudo dpkg --get-selections > installed-applications.txt
it will generate a list of installed applications in txt file in home folder.
To use this file to install the same applications you will have to do ( But before this please also add all the Third party Repositories as was present in older one)
sudo dpkg --set-selections < installed-applications.txt
sudo apt-get -y update
sudo apt-get dselect-upgrade
This is also possible through Gui way
sudo add-apt-repository ppa:webupd8team/mintbackup && sudo apt-get update
sudo apt-get install mintbackup
I guess image almost speaks itself, backup your selection, then in New installation restore it.