I always buy my son a second set of school textbooks so he can have a set at home, and at school, since about 5th grade, mostly from Amazon Used Book sales. Look for those sellers who live in your state or nearby, and have decent ratings. I have never had a problem. Usually the basic shipping (I think it's like $3.99 a book) has actually been much quicker than it's supposed to be, if I'm in a big hurry, then I'll pay the extra $3 or so from those sellers who offer priority shipping (or whatever it is).
I would be sure to match the IBSN #s to make sure you get the correct edition of the book, as there are changes from year to year. The sellers usually describe the books accurately, and will state if there are many marks, etc. Most books I've received are in at least the condition that was stated, if not better.