Here's why:
* Common nouns are general names for people, places, things, or ideas. They are not capitalized unless they are at the beginning of a sentence.
* Secondhand refers to something that has been owned or used by someone else. It's a general term for something that is not new.
Examples:
* I bought a secondhand car.
* The secondhand furniture was in great condition.
Let me know if you have any other questions about noun types!