So as in title, Is it possible to buy really large BTC amounts? like a million $? What was the largest amount of BTC you bought at once?
yes it is possible to buy large BTC if you buy in exchange you can look at the max withdrawal cause exchange have max withdrawal in a day, if you want to buy in same time prefer to separate your transaction in a couple exchange so you can withdrawal your BTC to your wallet in same time too , i dont know about the tax because i never buy bitcoin in a large amounts, but sorry do you have a million $ 