You could only sell it if someone was willing to buy it all-- there's no exchange that would buy $500,000,000 or more worth of Bitcoin at once. Alternatively,

You could only sell it if someone was willing to buy it all-- there's no exchange that would buy $500,000,000 or more worth of Bitcoin at once. Alternatively, he could slowly sell it off over time, but there's no way anyone could sell that much at once. Of course, he very well could be slowly selling it off right now.