Elon Musk’s sale of roughly $5 billion in Tesla Inc. shares in recent days is likely just the start of his selling wave.

When the Tesla chief executive took to Twitter over the weekend, pledging to unload 10% of his stock in the electric-vehicle company he runs, he owned roughly 170 million shares. Mr. Musk has since sold about 4.5 million shares through Wednesday, according to regulatory filings.