It was extremely common back in the 1500/1600s, when books were far more rare and valuable to write in them. Almost every copy of these truly old books has something written in them, often by multiple people as the owners changed. That's part of the history of the book.

It's not vandalism. Calling it that is condescending.

You can buy unmarked used books. Just check. (And if you're buying online the state of the book is expected to be in the description.)

vandalism is destruction of someone else's property. that's not what is happening when someone chooses to write in a book that they own.