If something is used, that means it has been owned by someone else. If you buy used textbooks, that means they’re not new — someone else owned them and read them before you did.
If you sell used cars, that means you’re selling cars that have already had at least one owner. Used can also describe things that have already served their purpose. Used batteries aren't good anymore: their power has been used up. People can also be used, and in this sense the word means “taken advantage of.” You might feel used by your neighbor if she's only nice to you when she needs you to water her plants.
