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Why invest for the long-term rather than buy and sell for quick, big gains?
A lot of people have already explained that your assumptions are the issue, but I'll throw in my 2¢. There are a lot of people who do the opposite of long term investing.  It's called high frequency trading.  I'd recommend reading the Wikipedia article for more info, but very basically, high frequency traders use programs to determine which stocks to buy and which ones to sell. An example program might be "buy if the stock is increasing and sell if I've held it more than 1 second."