One reason I think I like used books more than new, other than the 90% off, is the mark of the person who had it before me (and perhaps the few before that). I like seeing the pages they dog eared, the passages they underlined, the notes they made and sometimes even diagrams and such on the end papers. Those are a real treat. Those and notes like "HOG WASH" and an emphatic line drawn across the entire page.
Once I got a book, it was so tragic, with whited-out notes on just about every second page. "NOO!! When you did that you destroyed half the value of the book." And probably wasted a fair bit of time in the process. And it was the old fashioned white out that you had to wait for it to dry. So you can picture her brushing it on and holding it open and blowing on it before she could turn to the next page.
If it's your book treat it like it is your book. Let it show what is important to you. If you think it might be passed on perhaps avoid vulgarity in a children's book and keeping the spine healthy is a good idea. Missing pages get to me a little bit but I wouldn't worry about it.
Let people hear your critique if only one person at a time.