Most of the exchanges will ask for ID proof when you want to do transactions that big, you can proceed if you are buying legit way.
Like what you are telling about matter of trust to give ID proof to the exchanges to buy bulk quantity of bitcoins, then
it should be more cautiously do transaction in exchanges with this huge of buying as some of the exchanges can block the account after they get enough fiat currency .
So it is always to use a reputed exchange where high transaction are going on.
They may put a suspicious mark on you when you're trying to transfer that big amounts of money out of their banks.