Here's a wild thought: Some of the biggest companies in the world don't actually own  anything they sell. Airbnb doesn't own hotels. Uber doesn't own cars. Etsy doesn't make hand-knitted baby booties. And yet, these companies are worth billions of dollars. How? They're basically the world's fanciest matchmakers, introducing buyers to sellers and taking a cut every time someone swipes right (metaphorically speaking). Welcome to the marketplace model, aka the "we brought you tw