There are some really good CMS's out there that are free. My favorite being
http://www.e107.org. I operate 7 websites and 5 of them use e107. I would never pay for a CMS even if it did offer everything I was looking for. I would just customize it to my needs for free.
I don't have any suggestions of what kind of script to create and sell, but I think the best way to market it is to create two versions, a lite and a full. This will get people to use your script for free but if they want more features, they have to pay. I have looked back at the scripts I have purchased, and they all started by me using the free version first.
Just a suggestion!