If you buy a book, it is yours to do what you want with it. That is a given.
But if you are not planning on keeping a book and think about donating it or selling it, would it be too much to ask not to deface it?
I understand stains and the occasional page with a tear, but bent corners, cracked backs, highlights and writings are not things we like to see and deal with. Highlights and assorted marks distract me from what I am trying to do which initially is to get my own private lessons from the book. I do not need to know what you thought it was appropriate/interesting/necessary.
And for the love of all that is holy, any piece of paper and even a frigging toothpick make for an appropriate bookmark.