Even with public ledgers, you don't know if funds are moving to an exchange or between individual-owned addresses, especially when most exchanges allow you to generate new deposit addresses every time.
Also, exchanges like kraken allow "dark" trading, in which large sell walls by whales are invisible to other traders.
TL,DR: spotting a dump based on coin movements and sell walls is harder than we think.
The dark trading seems interesting to me, how do they allow this is this legal? Since these trades are not visible to the public then doesnt it mean that the fees there should be higher than normal or something like this? The kraken exchange is not user friendly as much as the poloniex or other similar exchanges but this dark trading seems a new thing to me which I would like to read more info about this.