If something is a good buy, it is of good quality and not very expensive.

If you buy into a company or an organization, you buy part of it, often in order to gain some control of it.

If you buy into an idea or a theory, you believe and accept it.

If you buy someone out, you buy their share of something such as a company or piece of property that you previously owned together.

If you buy up land, property, or a commodity, you buy large amounts of it, or all that is available.

If you buy something, you obtain it by paying money for it.

If you talk about the quantity or standard of goods an amount of money buys, you are referring to the price of the goods or the value of the money.

If you buy something like time, freedom, or victory, you obtain it but only by offering or giving up something in return.

If you say that a person can be bought, you are criticizing the fact that they will give their help or loyalty to someone in return for money.

If you buy an idea or a theory, you believe and accept it.