I would <3 Appshopper much more if you didn't have to create your owned-apps list manually. It's a total PITA IMO. I have many scores of apps and can't be bothered to do this. I'd even pay something for this feature.
It may be an Apple sandbox issue - apps may not be allowed to examine the app directory. If not though, I think this is a must-add feature for the app..
In the very first version of App Shopper when you would click on "Buy Now" it would auto put that app in your owned list. They have removed that feature - I wish they would bring it back.