"Then why wait for him to sell almost 40 million shares and make the price fall."
People are often asked this when it is possible that a large shareholder might be forced to sell for reasons that have little to do with the companies fundamentals.
Unfortunately there is no way to tell what effect Redstone's possible future forced sales would have on stock price of the company.
In the short-term the market is a voting machine. In the long-term its a weighing machine. Though there are people who make their living trying to predict how the market will "vote" on certain stocks in the short-term. I'm not one of them, and its likely you aren't either.
Hence in evaluating your investment in CBS you should only pay attention to developments that effect the long-term value of CBS's assets.
IOW, the advent of Tevo, is a much bigger threat to CBS than the forced selling of Sumner Redstone.
"Sell now and buy in after the stock takes the hit. He'll be done selling and CBS will be in the clear."
You are clearly a novice investor. Under the wash-sale rule you can't buy back shares you have sold in less than 30 days.
Besides this knowledge won't help you.
Everyone knows that Redstone has been forced to sell CBS shares and maybe forced to sell more. This information is already reflected in the stock price. His further selling won't necessarilly lead to a further decline in the stock.