Everybody states how they will sell their coins, but well before the price reaches $1,000,000, there would probably be a large amount of merchants accepting bitcoin due to its popularity. Why sell what you can use directly? Either bitcoin is used worldwide and is the top currency or everything else has inflated to such extremes that bitcoin is the only reasonable option in the event that the price reaches that high.
Even if starts getting accepted everywhere people will prefer to sell them as they also need physical money in their daily lives, and with real money you can invest them anywhere you want.