Your app's GUID doesn't match the GUID in the store. You have to take the published app guid from your store listing info (ie the one in the url for your app) and update your project files with this GUID.
For WP8.0 (Silverlight): WMAppManifest.xml
For WP8.1/Universal Apps: Package.appxmanifest
I found the solution for WP8.0 here, and the solution also works for Universal Apps. The problem manifests itself as a mysterious exception when calling "CurrentApp.RequestProductPurchaseAsync" or "CurrentApp.LoadListingInformationAsync". Very frustrating.
I just added a new data visualization/animation to my site, which animates a world globe synchronized with the Animaniacs song "Nations of the World".
Check it out! (though maybe not on a mobile device in the year that I am posting this). There's a link to the source code and project build info on that page.
This is my favorite Raymond Chen quote, and I'm putting it here so I can find it faster in the future:
Good advice comes with a rationale so you can tell when it becomes bad advice. If you don't understanding why something should be done, then you've fallen into the trap of cargo cult programming, and you'll keep doing it even when it's no longer necessary or even becomes deleterious.