Absolutely. If someone else (preferably rich) thinks that $500 doesn't justify true value of Apple, he could buy in the market, and good luck to short-sellers to borrow so many to sell.

My guess is that it's just some insider selling, and it's coincidental that it closed at $500 on option expiry date. Otherwise it would be hard to explain how it ended at $486 on Wednesday.

> is this even legal?

At least this is a stock, not a commodity like ahem oil.