A flash sale of his shares would also have wider financial implications regarding their value (so $ generated) and many other financial institutions linked to them.It's perhaps also worth considering that just over 2% of Bezos' net worth is in cash, with the remainder being in shares (presumably in Amazon itself). So in terms of cash, he's probably about as rich as Moshiri.
Bill Gates was asked why he hasn't gave all his money at once if he is so committed to philanthropy and his response was quite simple: they'd get much less.