http://www.homeschoolclassifieds.com This is the site I use to buy and sell my curriculum. I love it because if you don’t have a lot to sell you can sell 7 items for free and if you have a lot to sell it is very cheap per year. Check it out! There may be a book for sale that you had a hard time finding elsewhere!
donnayoung.org I love this site! It has so many resources for just about every aspect of home schooling, from calendar printables to multiplication drills to Science schedules. It is very helpful!
http://www.vegsource.com/homeschool/ I have used this website to look for and sell curriculum also. I have found some “hard to find” books on here! I have also sold on here, but not as often because you really need to repost your ad everyday for it to be seen the most often
http://www.ebay.com/ Ebay is also a good source to buy and sell curriculum, though I find I make a better profit through Homeschoolclassifieds
Hope these help you! As I find more I will share them!