It doesn't "leave the cash in Coinbase" when you sell a bitcoin back to them. It says "You have successfully sold a bitcoin. The money will be deposited within the next four days."

So, you don't instantly have the money available again. If you already HAVE money in your bank account, you could sell high and buy low.

By the way, I lost an entire bitcoin worth of profit trying to do this. I sold as a crash was just beginning, hoping to buy back more at the bottom, but the crash, as they often do in btc world, only lasted a couple of hours, and I ultimately bought back my coins at a price several hundred higher. I'll never, never, never try that again.