You should  buy low and sell high! This is well-known advice that means the best way to make money is to buy something (usually stocks) when the price is low and then sell it when the price is high. That way, you will make money.
In grammar, words like should, would, and could are called modals. Modals have special meanings and sometimes one modal has almost the same meaning as another. Look at the following:

                                      Could   you please sign your full name here?
                              Would   you please take this to customer service?

Both could and would are used for polite requests when you ask someone to do something. Study the following examples and reasons for using modals.

Polite Requests -------> would, could, can, mayy

As you have seen, would and could  can have the same meaning. Can and May also are used in requests. Look at these examples:

                  Can I see your identification?
                  May I help you?

Note: May is not used with "you" in requests.

Giving Advice --------> should, ought to, had better

When we give advice, we tell someone what we think is a good idea for them. Look at these examples of giving advice:

                           You should  deposit money in your account every month.
                           His broker said he ought to buy stock in a health-care company.
                           She had better pay off the first loan before she applies for a second one.

pay off   means finish paying
had better  is usually stronger in meaning

Necessity --------> must, have to

These modals are often used to state rules or regulations. Look at these examples:

A payment of $100 per month must be made on the loan.
When you withdraw money from an account, you must show identification or use a PIN number.
PIN   is a personal identification number used in electronic transactions.

When we use must not and don't have to, they have different meanings. For example:

        You must not sign someone else's name on a check. (Do NOT sign!)
       The customer doesn't have to disclose what he does with his money. (It's not necessary.)

 disclose  means to tell something, such as how much money

Making suggestions --------> could

Could   is also used to make suggestions or to tell what is possible. Here are some examples:

                            You could open an account with a check, or with cash.
                            The price of the stock could go up tomorrow.