I think, technically, it's not a violation. My recollection is that you aren't allowed to sell products through your app that are paid for outside of Google's in-app purchase system unless it is for a good or service that is consumed outside of Google's ecosystem. An example they give is of purchasing movie tickets through an app downloaded through Google Play (this would be permissible). As you aren't selling anything through the app (if the link is in the description) and what you are selling (the desktop app) is consumed outside of the ecosystem, I think you're safe. Of course, Google can change their policies without warning and they can also decide you are in violation arbitrarily.