My advice if you are trying to sell large crypto orders is use multiple exchanges to spread your risk. Coinbase and Gemini are insured up to $250,000 loses but that still does not cover a whales potential lost of in the millions of USD. Always use a cold storage wallet unit you must sell on these centralized exchanges. If we had decentralized exchanges now that were liquid crypto investors would not have this worry. With this said I think the risk of major hack are extremely low with insured exchanges. In any case loses would hopefully be paid back in full by the exchange like Bitfinex did when they got hack last summer.