In third grade, Grant LaFontaine sold a Pokemon card online. The buyer mailed a money order; Grant cashed it at the post office and shipped the card. Today he's co-founder and CEO of @Whatnot, where the largest businesses do over $100 million a year at up to 40% margins, and where users spend enough time to put the platform on par with social networks. In this conversation, Grant joins a16z's David George to discuss how that first transaction became a platform that did $8 billion in sales…