When homeschool parents need used books, they usually go to a homeschool classifieds site to find what they need. Some sites are better than others.
Some sites like vegsources.com can be difficult to navigate. All items are posted on a long list and you have to wade through the list and hopefully find what you need. I have not had much luck selling items there either because it was hard for people to look through what I had for sale.
On the other hand, homeschoolclassifieds.com is a great site. You can search for any title that you want. The site is pretty popular so there is a good amount of material available for sale.
If you want to sell books there it is easy to see what the going rate is a for a book or curriculum similar to the one you want to sell. You get 10 free listings and can pay a reasonable price to get more space for a year.
If you have a business, you can buy a business listing that is highlighted among the book listings. This is also a reasonable yearly fee.
Craigslist is becoming more and more popular on a local level. If you have a craigslist available for your city, chances are many people are using it to find what they want. You may be able to list your books there and have someone in the area purchase what you have for sale. They usually come and pick up the items, so you both save on shipping costs and the hassle of packing books for shipping.
Ebay is also a form of classified advertising. You can buy and sell homeschool books there. In this case the world is available to you. You don't have to just rely on your local market like Craigslist. Some people get good deals on homeschool books and resources on ebay and some think the prices go too high. It just depends on the time of year and the item being purchased.
If the item you are looking for is in high demand (like some Abeka titles), then chances are, you will pay close to full price on ebay. Also, if it is the end of August just before school starts, the price will tend to be higher because more people are purchasing homeschool books at that time.
As with any homeschool classifieds sites, always make sure you do your homework when buying or selling homeschool books. Pay close attention to the description and ask questions if the person doesn't list everything you need to know.
Many people like books from a pet-free, smoke-free home because of allergies. Others don't have allergies but don't like to open a book that has a strong pet or smoke odor. These are all things to consider when purchasing books.
Pay attention to whether the book has and markings or pages filled in, what the cover looks like, if all the pages are there, what edition the book is, and when it was published.
As long as you know what you are looking for and know the questions to ask, you should have good success when purchasing or selling items on homeschool classifieds.