All of you heard about big dump at GDAX exchange. Now i want you to look at what happened. GDAX as an exchange have their order book where they can clearly see what orders is in, as well stop loses. They knew exactly what amount needed to be sold in order to trigger massive stop loses cascade, so what they did is - they placed buy order all the way down - it could start from the price at which their last ETH was sold down to a 0.1$ or they could place all their buy orders at the price of a dollar or 2 . So what they did is they sold thousands of ETH at the price ranged from 220 - 350 and bought it back at ridiculously low price of just a dollar or 10$ or anywhere between 0.1 - 220$ as an outcome, they got their ETH back + millions of $ in profit just like that. And for an exchange, they dont need to have this amount of ETH at all , is only a numbers on their screen they could imitate the dump leaving all the profits for themselves. The point is : to make such operation, you need to know what is in the order book, and for sure you need to know all the margin orders as well their stop loses. Without this information it would be a huge risk to do (basically a lottery). So whoever did this, has such information, which give great grounds to suspect GDAX involvement in this operation, whatever was it direct or indirect.
NEVER PLACE MARGIN ORDERS !!! Not at crypto market, as this market (unlike fiat) is very easy to manipulate... Be safe !!!