OpenAI is in early discussions about a secondary stock sale that would value the artificial-intelligence developer at roughly $500 billion, according to people familiar with the matter. The planned transaction is aimed at allowing current and former employees to sell shares and could amount to several billion dollars. Existing investors, including venture firm Thrive, have approached the company about participating in the deal, the people said. An implied valuation of $500 billion would make OpenAI the world’s most valuable privately held company. On a public-market scale, it would rank about 16th among U.S. corporations—surpassing Netflix, Exxon Mobil and Costco—and stand at roughly half of Tesla’s market capitalization.