If I'm in Germany and buy a piece of software over the Internet, where does that sale take place? My current geolocation in the world, even though I could be driving down the road at the time? The server that hosts the "confirm" button click on the web site? The headquarters of the corporation I'm buying it from? The home office of the sales person who writes the contract?

All of those places can be on different continents from each other. Who has jurisdiction to tax? All of them, divided equally? Just the purchaser as a sales tax? The publisher, as a revenue tax? Both?