I just thought of something...

What about offering a "Lite" version of the theme and then a PRO version.

zeebizcard is an example. I just downloaded it (I had it before), When you go to Theme Options --- there is a Check out ZeeBizCardPro and the box next to it says Join the Theme Club.

What do you think of this Lite/PRO option? Here is what you find in the Check out ZBCP box:

"The PRO Version provide additional custom widgets and an author box."

+ Recent Posts Widget (with thumbnails)
+ Tabbed Widget (can show 8 different content types)
+ Recent Comments Widget
+ Popular Posts Widget (with thumbnails)

Do any of you remember in the 1990s the SHAREWARE 3.5 floppy disks thing?
I would get Duke Nukem 1 for free (cost of 3.5 floppy disk) then DN 2/3 would be paid.