Underline your textbooks with pencils instead of highliting them with markers/highlighters

For those of you who are in college or university and highlight the textbooks you bought, you probably won't be able to sell them back because they are highlighted and cannot be removed. Pictured below is what I mean:

Once there is highlight on it, the bookstores most likely won't buy them back because of this. But if you use pencil to underline important text, you can later erase if after your course(s) is over and sell it back to the college or university bookstore.

By selling back books, you get them resold, reused and recycled. But if you highlight it, you can only use it once and may have to keep it, thus wasting paper resources.