How much btc do you think you need before you can move the price on a exchange?
Also is manipulation legal at bitcoin exchanger?
Just find the exchanger provide a massive trading amount everyday. As I know , the high volume exchanger were poloniex,bittrex, and Bter.
About manipulation, that's can't be done with one person. But it can be done with a group of whale trader
you don't need to be in a group in order to manipulate the price. one person is enough. most important is to have loads of fiat and bitcoins.