One of the ways I stay within our budget is by selling any homeschool books we no longer need and using that money to purchase curriculum for the next year (used if possible). I’ve collected quite a few links for buying & selling online, and I hope some of them have just what you need!
A-Z Home’s Cool Homeschool Classifieds Yahoo group
PaperBack Swap (this is my referral link)
Half Price Books (not online, but a great place to look)
Blog it: Do you have a blog? Post a Books for Sale list! You can promote it to your readers, and you will also get sales from people doing searches for specific books.