I'd like to be able to create and sell PDF formatted maps online. 
Users would select an area by zip code, county, state, or CBSA and a
map would be generated showing all zip codes in that area.  An example
of what I'm looking at is here:
http://www.zip-codes.com/digital-zip-code-maps.asp . 

I guess my question is - what software do I need and what data files
are needed to generate the maps?

Any help would be greatly appreciated.

you would need mapping software primarily.  take a look at arcims
arcgis server from  www.esri.com.  As far as the customized maps go
you would need someone to write a webapplication to talk to the map
server to generate the images and create a pdf from that.