Whether or not the condition of a used book is acceptable depends on your standards and your intended use of it. For example, if you want to give it as a gift, the book should not be falling apart, stained or be missing pages. On the other hand, if you just want it for your own personal use, you may be able to tolerate a worn cover, stains on the cover, and pages that need a little tape to hold them in the book.
When I was a university student, I often bought used books instead of new ones to save money. Many times I even liked the notes people had made in the textbooks before I got them. Some of them were quite helpful to me.
However, I do have to add that some of the books I bought from used bookstores were musty. Since I am sensitive to dust, must, and mold, I sometimes got a headache from reading these. If you are even more sensitive to this than me, smell or have someone else smell the book before you buy it. Exposing the book to sunlight may lessen the mold problem, or you can make sure you read it just a little at a time and not when you're already tired or sick. I found fatigue or already being tired tended to decrease my tolerance for this allergen.
To be the most valuable, an antique book should be in almost as good a condition as the day it came off the printing press. A book that is extremely rare can be a little more beat up because it is so hard to find. When Amazon lists used books on its website, it always rates their condition from "Like New" all the way down to "Acceptable w/some marking, stains," etc. The better copies are usually pricier. I really appreciate this very detailed and honest assessment. Thank you, Amazon.