You can sell your gadgets, widgets, themes, etc.
The ONLY Limitation is that you may not build a desktop and then export it as a GADGET (.EXE).
So for instance, we made "Natural Desktop" (www.naturaldesktop.com). We don't want someone to buy DesktopX Pro for $70 and start cranking out kiosks, desktops, etc. with it. We make a product for that called DesktopX ENTERPRISE ($500).
Gadgets should be programs, not desktops.
But other than that one limitationk, you can go nuts.
We are even setting up a site www.desktopgadgets.com to fasciliate people selling their stuff.