Neither Buying at High Price nor Selling at Low Price.

The eager buyers of shares pushed prices to dangerously high levels, so by 1970, most stocks were fatally overpriced.  By almost any measure, people were paying far too much for the companies they were buying. 

This sort of craziness happens a few times in a century, and whenever it does, the market "correct," the prices drop to more sensible levels, and the people who bought at the top are stunned and depressed.  They can't believe they've lost so much money so quickly.

Of course, they haven't really lost anything unless they sell their shares, but many investors do just that.  They dump their entire portfolio in a panic.  A stock they acquired for $100 when it was overpriced, they unload a few weeks later for $70 or $60, at a bargain price.

Their loss is the new buyers' gain, because the new buyers will make the money the sellers woulod have made if they'd held on to their investments and waited out the correction.