Hey Peter - just wanted to chime in since I'm your target market (I run the wordress plugin AmpedSense).
One of the difficulties about selling a plugin outside of the wordpress plugin directory is the inability to push updates easily, so you're definitely hitting something here with me. Although the more I look into your service, the more I realize it's not for me, as I'd hate to ask my customer to install 2 plugins (mine and the updater).
What would be interesting to me is if there was a module I could embed into my plugin that could take care of updating itself. Who knows if that's even possible, but thought my perspective might help...
Thanks for sharing, and good luck!