Trialware could be tricky to integrate into the current management, but surely free demos would be simple to do? Like the free apps we see already on ITMS? Where you've got so many titles with "xyz LITE" and "xyz full/pro".
And a lot of the time the only difference between the LITE and full versions is one is ad supported and the other is bought and has no ads.
I work for the Department of Redundancy Department
