A Harry Potter T-shirt was maybe never going to sustain an empire. Or coasters shaped like national landmarks, or Christmas ornaments with your dog’s face on them. Empires are built on unlikely foundations. Romulus built Rome on shards of Etruscan pottery and wolf milk. Cecrops built Athens on a hill and a lot of goat meat. And between 2005 and the fall of 2021, Etsy built a $37 billion empire on personalized coasters, dog-themed Christmas ornaments, and bootleg Taylor Swift merch. On November 2...