You buy stocks for two reasons. Either you think they will go up (long) or you think they will go down (called "shorting"). You can think of them as opposites.
Buying short isn't the opposite of buying long - it's the reciprocal. Which is an important distinction: if you buy a diversified portfolio of 10 stocks and one goes bust, if you bought long you've lost 10%. If you bought short, you're completely broke.
If your initial hunch is to buy a stock you don't own yet, you can't sell it instead.