Hah! I had the exact same idea. I created a landing page, posted it on HN and asked:

1. "Is my startup idea legal?" https://news.ycombinator.com/item?id=7167200

2. "Is it possible to 'shop without borders'? https://news.ycombinator.com/item?id=7166333

Interesting discussion ensued. Read more about my tiny non-existent precursor to Backpack :) I am so intrigued by the fact that they actually went ahead, launched and executed! Hats off. I never progressed beyond just thinking about it.