Three friends who met in a sneaker reselling Facebook group in middle school just got Sequoia to value their company at $5.1 billion. Sequoia actually turned them down the first time. Today it led their $200M round, and Doug Leone, the guy who used to run the firm, picked it as his first deal since coming back to investing. The company is Whop, and it started as a way to sell sneaker bots. Steven, the CEO, figures that whole market was worth maybe $20,000 a day. That's about $7.3M a year,…