Could sell and buy back cheaper later

Very often, you would heard someone said "Could sell and buy back cheaper later".

Sometimes, someone may also said "If I should have sold it at at x price, and buy back at x-0.5 price. Then I would gain xxx instead of lose yyy."

However, this is all based on hindsight.

Irony is, how many times did the share went up further after you sold? How many times did the share crash after you decide to hold it and think it would go up further? How many times did share you bought drop more just after you just bought it?

If it is so easy to predict the up and down of the market, all of us would be billionaires and don't need to work right?

If you are so zhun, can you please predict and tell me when the market will crash and at which date will it rebound back?

If you are so zhun, can you please predict and tell me if the market will U-turn given the current bad economic conditions?

One classic advice:

Slowly buy, slowly sell. You can never sell at the exact top or buy at the exact bottom - always buy and sell in stages. (The top tick of a rally is the most expensive tick inthe market - fortunes have been lost hunting for it.)