Hi, i'm also making a content management system, which will be good - I hope. It's not 'demo' ready yet but should blow PHPNuke right outta the water, which is my main reason for making my work free!
I don't worry about anyone stealing my work and selling it as I plan to make my free version good enough and well known so that anyone buying a CMS will surely have heard of mine and would spot any similarities straight away.
My advice is that if you are real serious about selling your work, sell it lock stock to a PHP development company. Find one that'll give you a good price and you can work on something else while they worry about marketing and piracy!
Matt...