Instant cash if you already own bitcoin?

ref yahoo finance news here:
https://www.yahoo.com/finance/news/bitcoin-soars-more-3-000-170900407.html

"At 12:48 p.m., for instance, bitcoin was trading at $19,141 a coin on Bithumb, an exchange in South Korea. At the same time, it was trading at $14,573 on Bitfinex, based in Taiwan."

Agree, this is extreme difference in price, but I have noticed a few hundred (400-600) $$ difference in price of a bitcoin especially on a volatile day. And you know it gets volatile frequently.

For example, this morning after crash at gdax above, for a considerable amount of time (at least 30 minutes to an hour), bitcoin was trading on gdax for 400-600 $ more than gemini.

If one already own a bitcoin (or fraction) at gdax and have cash ready at gemini, can make instant cash by buying at gemini and selling at gdax at the same time and pocket the difference!

Or am I dreaming??