Saving the list of applications in Synaptic is dead simple. File, Save
markings. Check the box at the bottom to save Full state (not just
changes). Put the file somewhere safe in your home directory if you have a
separate home or on an external device otherwise. Post installation, it is
Read markings and navigate to the file. I always update my sources list
first, otherwise it will only install packages in the repositories and
ignore external sources, like medibuntu, get-deb, Google, PPAs.

You can do the same from the commandline, but I hate navigating folders and
drives from the commandline and then have to move it with the file manager
after which seems to be more work.