"it's manipulation for big investors to sell out and make some profit while they can before the stock tanks."
Perhaps. But if you study actual investing performance of "Big Investors" such as hedge funds, they don't do particularly well. Most of the wealth techniques Buffett uses can be used by anyone. As for brokerage houses, many, many of them have failed over the decades. That's something Ben Graham points out in The Intelligent Investor.
We always hear of the ultra wealthy hedgies, but every year tons of hedge funds fail... quietly.