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.