This happens pretty much every time there is a major move. If you like Coinbase you need to transfer fiat to GDAX and place low ball buy orders. You can't count on Coinbase being responsive during major market moves but if you have buy orders in at GDAX they should get filled.
I am only using the regular exchanges like yobit, poloniex etc where still the trading is going on, coinbase like services are waste as when they see high dump or pump they just close their server so that traders should not get the benefit of this crisis. It is so unfair policy of this type of exchanges where they are controlling the site according to their wills.
Like stated above, selling coins at Coinbase is directly to their wallets. It's like our local exchange wallet here where once we sold our coins, we are dealing with the company and not on other trader itself like those in normal trading exchanges and example of them are the exchanges you mentioned.
It's really unfair for those who have an active funds at Coinbase since they can't execute selling their coins during price crash. And now no profit taking happened because they are not operational.
