My favorite place to find and sell used homeschool curriculum is at homeschoolclassifieds.com. I have found some things at ebay, but the prices there tend to go a little high. I also enjoy going to used curriculum fairs in my area so that I can actually look at the books I am thinking of buying.
Goodwill/Salvation Army/Thrift Stores - You would be surprised what kinds of things you find here. I have found Abeka books, great readers, unusual educational toys and much more.
Used Homeschool Curriculum Fairs - I have a favorite one I go to each year in May in my area. It is a Christian school that is homeschool friendly. They have a homeschool division and let homeschoolers sell their own books at the fair. The best part is that they also sell off books and resources from the school and have a whole bunch of free stuff out in the lobby that you can just take.
Garage Sales - Although not as good as the above 2 choices, I have found mostly homeschool supplies, readers and educational toys at garage sales. These all add to the depth of my homeschool curriculum.
Library and Library book sales - Several libraries in the area have sales of books that they no longer need. They also take donations from individuals and sell those books too. You can also get puzzles, games and magazines at these sales. Usually the prices are very low. I also use the library interlibrary loan for books that I only need for a few weeks. Our library system is all on the internet. So if I find a book that I want my children to read (or read to them), instead of purchasing it I request it online through interlibrary loan. It's wonderful because they get the book for me and have it delivered to my local library for me to pick up. It's a great system!
What to Look for When You Buy Used Homeschool Curriculum:
When you go to buy used homeschool books or curriculum, there are a few tricks you should know of what to look for when you are looking at a used book.
Make sure you know which edition you are looking for if that matters to you. Make sure all the pages are there and note any markings inside. If you want a totally clean workbook so your child can fill it in on their own, then be sure to look for that. If you don't really care what is written in the book, then you should be able to get a good price.
When you go to a book sale, you can look for these features yourself, but if you are purchasing online, you will need to ask these questions if the seller hasn't mentioned it. I always try to describe my books very carefully so that the seller knows exactly what they are getting. Other sellers either don't know to do this or don't take the time to inform their buyers. I always like for my buyers to get a book and be surprised that it is in better condition than they thought it would be.
Used Abeka Books
If you are looking for used Abeka books be sure to check my guidelines for those as their editions change often. Make sure this is the curriculum you really want. Lots of parents think that Abeka curriculum is the only good curriculum, when in fact there are many good homeschool curriculum options, some even better than Abeka.
Many parents sell their used Abeka books on Amazon. You can purchase books right away without having to wait for an auction to finish. You can see a picture of the book and get a good description of what you are getting. Sometimes there are even reviews of the book there.
When to Buy Used Homeschool Curriculum and Books
Since most parents start thinking about what they want to buy for next year once the school year is over, the summer tends to be a high volume time for purchasing new and used books. The demand is high in the summer so the prices are also a bit higher.
If you can purchase books between November and March, you will probably find used homeschool curriculum and books at a reduced cost. Once you start purchasing when everyone else is purchasing, the prices won't be as good as in the off peak months.
As always, there are plenty of homeschool books and resources available at ebay and Amazon. Ebay prices are a little bit better for sellers than buyers, but you can still find almost everything you need there and sometimes for a good price. Another advantage to Amazon is that you don't have to wait for an auction to finish. You can buy and sell books there right away and usually pay less fees.