I just discovered Sharify, an interesting service which goal is to provide AIR developers the tools for create shareware (sellable) applications.
It seems also very easy to use, the steps are: register to the site, create an application id, integrate the sharify library (an swf file) into the application and choose a payment method (like paypal). The service unfortunately is not free, but require the 3% of each license sold… however I find the percentage very reasonable (if the service is valid and secure as I expect).
I didn’t try it yet, but I think I’ll do in the future :)