DETROIT (AP) - Elon Musk is selling more Tesla shares than he needs to pay current tax obligations, and experts say he's either converting part of his fortune from stock to cash, or he's saving for bigger tax bills that will come due next year.As of early Wednesday, Musk had sold roughly 8.2 million shares in the electric car and solar panel maker in the past nine days, worth a total of just over $8.8 billion.Of those, Musk sold 2.8 million shares worth about $3 billion specifically to pay taxes on three tranches of stock options that he exercised this week, according to filings with the U.S.