I have listed some tips on how to buy used homeschool books and curriculum over on my website. There are lots of places to buy used curriculum and books. Just because you purchase from a reputable place doesn't mean you will get a good deal or a good book.
Sometimes people online don't know how to describe their books accurately. Other times people have so many books to list that they don't have time to describe them all accurately. If you have any type of pet or smoke allergy, it's important to ask about that if you are looking at a homeschool book or curriculum. Sometimes the person selling the books forgets to mention if they have pets or smoke because they are not aware that it can be an issue for some people.
I've listed exactly what to look for and when to buy. There are plenty of places to buy books whether you purchase from a friend, a used book sale in your area, or online. Many of us homeschool for free or almost free just by buying and reselling used homeschool books.
How to Buy Used Homeschool Books