Do I correctly understand that Google Marketplace does not tell you who has bought or returned your app?

I'm contemplating a low-cost, one-time for-pay service, and since the service will also be accessible outside the G1, I need to know how to grant access to users who bought the Marketplace app, and deny access to users who returned it.

Is there perhaps some mechanism for querying Marketplace to see if a particular user already owns a specific app?