This is why "condition" exists for secondhand books. People can choose a written-in one or not. If you can't afford new, note that written-in ones are often even cheaper!

I have been delighted by the writing I've seen in books, especially if it's a copy of a friend or family member, as you feel you are connecting to another reader years later, and the item is no longer identically mass-produced. But maybe it depends on the taste of the person writing in the book.

I think the truth is most printed copies of books are rarely read even once, so the idea of preserving them is noble but realistically not that important. And may be outweighed by benefits accrued to the person currently owning and reading it making the most of it and marking to their hearts content.

I admit I don’t mark up my books but I feel tempted to try

>I would even say that "crossing out lines you disagree with" is vandalism

if you cross out words in a book you borrowed, that is vandalism. if you cross out words in a book you own, by definition, that is not vandalism.

it would be uncool to mark up a textbook you planned to resell at the end of the semester, but it still would not be vandalism assuming you bought the textbook.

Again, depends… I have appreciated my seniors’ notes in some textbooks - especially where the book itself isn’t clear.

> Well not everyone has the privilege to buy new books, especially course-mandated books that the professor "coincidentally" wrote and costs hundreds of dollars.

Do you even understand what you sound like? "Writing in something you own is vandalism because poor people".